Recursos intangíveis de um destino turístico: um estudo de desempenho da cidade de Natal tendo como base a teoria das capacidades dinâmicas

O objetivo desta tese foi analisar os recursos intangíveis de um destino turístico. Foi feito um estudo de desempenho da cidade de Natal tendo como base a teoria das capacidades dinâmicas. Inicialmente, foi desenvolvido um instrumento de pesquisa para oportunizar a construção de um modelo de medida com vistas à captação de variáveis latentes para os recursos intangíveis existentes no setor do turismo em Natal-RN. Em seguida, foi realizada uma análise dos recursos intangíveis que são evidenciados pelos turistas acerca da cidade. Finalmente, foi feita a investigação de um modelo de estrutura que estabeleceu as relações entre as variáveis latentes dos recursos intangíveis e a percepção de desempenho do turismo na cidade de Natal. Procedeu-se uma revisão de literatura para construção de um modelo inicial acerca dos ativos intangíveis dentro da teoria das capacidades e foram relacionados dez recursos, quais sejam: recursos humanos prestadores de serviços; recursos humanos como gestores; cultura local; conservação do meio ambiente; know-how empresarial; inovação empresarial; tecnologia; marca; preço; e promoção. Estes dez recursos, em conjunto, seriam responsáveis pelo desempenho do destino turístico. Após ser realizada uma análise de equações estruturais, apenas quatro recursos manifestaram relações com o desempenho: marca; cultura; conservação do meio ambiente; e preço. Análisando dos dados, verificou-se que a variável desempenho positivo da cidade na ótica do turista se manifestou fortemente e foi influenciada de forma forte pela cultura local, marca e conservação do meio ambiente em conjunto. O preço foi influenciado pelo desempenho positivo mostrando que o turista sente que o preço pago foi satisfatório diante dos atributos do destino Provavelmente, isso seja um ponto positivo muito forte para o destino turístico de Natal, pois, em termos de recursos intangíveis e de juntos formarem capacidades, eles são inimitáveis, raros e são capazes de se ajustarem às mudanças organizacionais e ambientais, para reconfigurar os ativos e as estruturas de um destino, corroborando, assim, com a teoria das capacidades dinâmicas. Esses recursos da cidade de Natal, como destino turístico, são únicos e provavelmente têm um peso muito maior para o desempenho da cidade do que problemas existentes na localidade. Logo, esses atributos devem ser estimulados a continuar crescendo e se modificando de acordo com as exigências atuais e futuras de consumo

Teoria de carteiras e a alocação de parques eólicos offshore

The consumption of energy on the planet is currently based on fossil fuels. They are responsible for adverse effects on the environment. Renewables propose solutions for this scenario, but must face issues related to the capacity of the power supply. Wind energy offshore emerging as a promising alternative. The speed and stability are greater winds over oceans, but the variability of these may cause inconvenience to the generation of electric power fluctuations. To reduce this, a combination of wind farms geographically distributed was proposed. The greater the distance between them, the lower the correlation between the wind velocity, increasing the likelihood that together achieve more stable power system with less fluctuations in power generation. The efficient use of production capacity of the wind park however, depends on their distribution in marine environments. The objective of this research was to analyze the optimal allocation of wind farms offshore on the east coast of the U.S. by Modern Portfolio Theory. The Modern Portfolio Theory was used so that the process of building portfolios of wind energy offshore contemplate the particularity of intermittency of wind, through calculations of return and risk of the production of wind farms. The research was conducted with 25.934 observations of energy produced by wind farms 11 hypothetical offshore, from the installation of 01 simulated ocean turbine with a capacity of 5 MW. The data show hourly time resolution and covers the period between January 1, 1998 until December 31, 2002. Through the Matlab R software, six were calculated minimum variance portfolios, each for a period of time distinct. Given the inequality of the variability of wind over time, set up four strategies rebalancing to evaluate the performance of the related portfolios, which enabled us to identify the most beneficial to the stability of the wind energy production offshore. The results showed that the production of wind energy for 1998, 1999, 2000 and 2001 should be considered by the portfolio weights calculated for the same periods, respectively. Energy data for 2002 should use the weights derived from the portfolio calculated in the previous time period. Finally, the production of wind energy in the period 1998-2002 should also be weighted by 1/11. It follows therefore that the portfolios found failed to show reduced levels of variability when compared to the individual production of wind farms hypothetical offshore

Processo de implantação de um sistema integrado de gestão em uma organização pública à luz da teoria da hospitalidade :um estudo de caso

In the present time, public organizations are employing more and more solutions that uses information technology in order to ofer more transparency and better services for all citizens. Integrated Systems are IT which carry in their kernel features of integration and the use of a unique database. These systems bring several benefits and face some obstacles that make their adoption difficult. The conversion to a integrated system may take years and, thus, the study of the adoption of this IT in public sector organizations become very stimulant due to some peculiarities of this sector and the features of this technology. First of all, information about the particular integrated system in study and about its process of conversion are offered. Then, the researcher designs the configuration of the conversion process aim of this study the agents envolved and the moments and the tools used to support the process in order to elaborate the methodology of the conversion process understood as the set of procedures and tools used during all the conversion process. After this, the researcher points out, together with all the members of the conversion team, the negative and positive factors during the project. Finally, these factors were analysed through the Hospitality Theory lens which, in the researcher opinion, was very useful to understand the elements, events and moments that interfered in the project. The results consolidated empirically the Hospitality Theory presumptions, showing yet a limitation of this theory in the case in study

Estudo do crescimento, eficiência de biofiltração e cinética de absorção de nutrientes (N-NH, N-NO e P-PO4³) da macroalga Gracilaria cervicornis (Turner) J. Agardh

The objective of this study was to examine the growth of Gracilaria cervicornis cultured in a shrimp (Litopenaeus vannamei) pond and to determine the absorption efficiency and the kinetics parameters (Vmax, Ks e Vmax:Ks) of this macroalgae for the nutrients N-NO3-, N-NH4+ and P-PO4-3, aiming at its use as bioremediatory of eutrophicated environments. For this study, two experiments (field and laboratory) were developed. In the field study, the seaweed was examined in relation to the growth and the biomass. In the laboratory experiment, the absorption efficiency of G. cervicornis was measured through the monitoring of the concentration of the three nutrients (N-NO3-, N-NH4+ e P-PO4-3) during 5 hours and the kinetic parameters were determined through the formula of Michaelis-Menten. The results obtained in this study demonstrated that G. cervicornis benefited from the available nutrients in the pond, increasing 52.4% of its biomass value after 30 days of culture. It was evidenced that the variability of the biomass could be explained through the salinity, availability of light (transparency and solid particle in suspension) and concentration of N-NO3- in the environment. In the laboratory experiment, the highest absorption efficiency was found in the treatments with low concentration (5 µmol.L-1), being evidenced a reduction of up to 85,3%, 97,5% and 81,2% of N-NH4+, N-NO3- and P-PO43-, respectively. Regarding the kinetic parameters, G. cervicornis presented better ability in absorbing N-NH4+ in high concentrations (Vmax = 158,5 µmol g-1 dry wt h-1) and P-PO43- in low concentrations (Ks = 5 µmol.L-1 e Vmax:Ks = 10,3). The results of this study show that G. cervicornis could be cultivated in shrimp ponds, presents a good capacity of absorption for the tested nutrients and is a promising candidate for biorremediation in shrimp pond effluent

Síntese, caracterização e cinética da decomposição térmica dos materiais cerâmicos PrMO3 (M = Ni ou Co)

In this work have been studied the preparation, characterization and kinetic study of decomposition of the polymerizing agent used in the synthesis under non-isothermal condition ceramics PrMO3 of general formula (M = Co and Ni). These materials were obtained starting from the respective metal nitrates, as a cations source, and making use of gelatin as polymerizing agent. The powders were calcined at temperatures of 500°C, 700°C and 900°C and characterized by X-ray Diffraction (XRD), Thermogravimetric Analysis (TG / DTG/ DTA), Infrared Spectroscopy (FTIR), Temperature Programmed Reduction (TPR) and Scanning Electron Microscopy (SEM). The perovskite phase was detected in all the X-rays patterns. In the infrared spectroscopy observed the oxide formation as the calcination temperature increases with the appearance of the band metal - oxygen. The images of SEM revealed uniform distribution for the PrCoO3 and particles agglomerated as consequence of particle size for PrNiO3. From the data of thermal analysis, the kinetics of decomposition of organic matter was employed using the kinetics methods called Model Free Kinetics and Flynn and Wall, in the heating ratios 10, 20 and 30° C.min-1 between room temperature and 700°C. Finally, been obtained the values of activation energy for the region of greatest decomposition of organic matter in samples that were determined by the degree of conversion (α)

Estudos de Durabilidade Frente ao Ataque Ácido de Compósitos Portland-Polímero para Cimentação de Poços de Petróleo

Portland-polymers composites are promising candidates to be used as cementing material in Northeastern oil wells of Brazil containing heavy oils submitted to steam injection. In this way, it is necessary to evaluate its degradation in the commonly acidizind agents. In addition, to identify how aggressive are the different hostile environments it is an important contribution on the decision of the acidic systems to be used in. It was investigated the performance of the Portland-polymer composites using powdered polyurethane, aqueous polyurethane, rubber tire residues and a biopolymer, those were reinforced with polished carbon steel SAE 1045 to make the electrochemical measurements. HCl 15,0 %, HCl 6,0 % + HF 1,5 % (soft mud acid), HCl 12,0 % + HF 3,0 % (regular mud acid) and HAc 10 % + HF 1,5 % were used as degrading environment and electrolytes. The more aggressive acid solution to the plain Portland hardened cement paste was the regular mud acid, that showed loss of weight around 23.0 %, followed by the soft mud acid, the showed 11.0 %, 15.0 % HCl with 7,0 % and, at last the 10.0 % HAc plus HF 1.5 % with just 1.0 %. The powdered polyurethane-composite and the aqueous polyurethane one showed larger durability, with reduction around 87.0 % on the loss of weight in regular mud acid. The acid attack is superficial and it occurs as an action layer, where the degraded layer is responsible for the decrease on the kinetic of the degrading process. This behavior can be seen mainly on the Portland- aqueous polyurethane composite, because the degraded layer is impregnated with chemically modified polymer. The fact of the acid attack does not have influence on the compressive strength or fratography of the samples, in a general way, confirms that theory. The mechanism of the efficiency of the Portland-polymers composites subjected to acid attack is due to decreased porosity and permeability related with the plain Portland paste, minor quantity of Ca+2, element preferentially leached to the acidic solution, wave effect and to substitute part of the degrading bulk for the polymeric one. The electrolyte HAc 10 % + HF 1,5 % was the least aggressive one to the external corrosion of the casing, showing open circuit potentials around +250 mV compared to -130 mV to the simulated pore solution to the first 24 hours immersion. This behavior has been performed for two months at least. Similar corrosion rates were showed between both of the electrolytes, around 0.01 μA.cm-2. Total impedance values, insipient arcs and big polarization resistance capacitive arcs on the Nyquist plots, indicating passivity process, confirm its efficiency. In this way, Portlandpolymers composites are possible solutions to be succeed applied to oilwell cementing concomitant submitted to steam injection and acidizing operation and the HAc 10,0 % + HF 1,5 % is the less aggressive solution to the external corrosion of the casing

Sensibilidade da cinética do zinco e avaliação nutricional em crianças submetidas ao teste venoso de tolerância ao zinco

Objective: The purpose of this study was to investigate the kinetics of this micronutrient in schoolchildren between the ages of 6 and 9 years, of both sexes, and to verify its sensitivity in detecting alterations in body zinc status. Methods: Nutritional assessment was performed by body mass index. Food intake, venous zinc tolerance test and zinc kinetics were carried out before and after 3-month oral zinc supplementation. Results: Of the 42 children studied, 76.2% had healthy weight. Only energy, calcium and fiber intake were suboptimal before and after oral zinc supplementation. Serum zinc and total-body zinc clearance, although at normal levels, increased significantly after zinc supplementation. Conclusion: We concluded, therefore, that kinetics is a sensitive tool for detecting changes in body zinc status, even in children without a deficiency of this mineral. Furthermore, kinetics showed a positive response to supplementation and may be a sensitive parameter to evaluate the efficacy of this therapy

A teoria democrática e a participação popular : as trajetórias participativas no Recife e em Módena

This work basically achieve three goals. Critically investigate the liberal democratic regime and its historical reformulation, rejecting the popular power and popular self-organization, limiting the entry of normal citizen in decision-making, believing in the market as a mediating body in regulating of the different life spheres of social. Starting from the critical liberal democracy, it discussed the concept of popular participation in the democracy, searching new democratically horizons, where the masses could have the opportunity to make decisions about their own destiny. On the basis of theoretical discussion on participation, we discuss a concrete instrument of participation, the Participatory Budgeting, comparing two participatory experiences in North and South

A Eficácia dos direitos fundamentais nos contratos civis e de consumo: uma teoria contratual baseada na colisão dos direitos ou bens constitucionalmente protegidos

The apparent virtuosity that if could wait of the globalization and the neoliberalism has given signals of deterioration in the contractual relations, especially in contracts of mass consumption, generating innumerable offensive situations to the basic rights and the goods constitutionally protected of the contractors. In the world of today, still that it does not reveal any desire, the individual practically is compelled to contract, for force of necessities and customs completely imposed, mainly in face of the essentiality of the services or agreed to goods. Ahead of as much and unexpected changes in the civil liames and of consumption, dictated for the globalization, it comes to surface the reflection if the private law e, more specifically, the civil law, meet prepared adequately to deal with these new parameters of the economy. The present dissertation has the intention to investigate if the globalization and the consequent neoliberalism, in this beginning of third millennium, will imply to revive of the principles and the basics paradigms of the contracts that consolidated and had kept, for more than two centuries, the liberal State. One notices that the study of this phenomenon it gains importance to the measure where if it aggravates the decline of the social State (Welfare State), with the embrittlement and the loss of the autonomy of the state authority, over all in countries of delayed modernity, as it is the case of Brazil, that presents deep deficiencies to give or to promote, with a minimum of quality and efficiency, essential considered public services to the collective and that if they find consecrated in the Federal Constitution, as basic rights or as goods constitutionally protecting, the example of the health, the education, the housing, the security, the providence, the insurance, the protection the maternity, the infancy and of aged and deficient. To the end, the incidence of constant basic rights of the man in the Constitution is concluded that, in the process of interpretation of the right contractual conflicts that have as object rights or goods constitutionally proteges, in the universe of the globalized perhaps economy and of the neoliberalismo, it consists in one of the few ways - unless the only one - that still they remain to over all deal with more adequately the contractual relations, exactly that if considers the presence of clauses generalities in the scope of the legislation infraconstitutional civil and of consumption, front the private detainers of social-economic power. To be able that it matters necessarily in disequilibrium between the parts, whose realignment depends on the effect and the graduation that if it intends to confer to the basic right in game in the private relation. The Constitution, when allowing the entailing of the basic rights in the privates relations, would be assuming contours of a statute basic of all the collective, giving protection to the man against the power, if public or independently private

Teoria da despesa pública: uma leitura ético-constitucional em busca do possível da reserva

Brazilian law passes through a crisis of effectiveness commonly attributed to the extravagance of fundamental rights and public shortage. However, public finances are not dogmatically structured to solve the conflicts around the limitations of public spending. There are ethical conditioning factors, like morality, proportionality and impartiality, however, these principles act separately, while the problem of public shortage is holistic. Also, the subjectivity of politics discretionary in the definition of public spending, which is supported in an indeterminate concept of public interest, needs material orientation about the destination of public funds, making it vulnerable to ideological manipulation, resulting in real process of catching rights. Not even the judicial activism (such as influx of constitutionalism) is shown legally appropriate. The Reserve of Possible, also presents basic ethical failure. Understanding the formation of public shortage is therefore essential for understanding the crisis of effectiveness of state responsibilities, given the significant expansion of the state duty of protection, which does not find legal technique of defense of the established interests. The premise of argument, then, part of the possibility of deducting minimal model ethical of desire to spend (public interest) according to objective parameters of the normative system. Public spending has always been treated disdainfully by the Brazilian doctrine, according to the legal character accessory assigned to the monetary cost. Nonetheless, it is the meeting point between economics and law, or is in the marrow of the problem of public shortage. Expensive Subjects to modernity, as the effectiveness of fundamental rights, pass necessarily an ethical legal system of public spending. From the ethical principles deducted from the planning, only the democratic principle guides the public spending through the approval of public spending in the complex budget process. In other words, there is an ethical distancing of economic reality in relation to state responsibilities. From the dogmatic belief of insufficiency, public spending is evaluated ethically, according to the foundations of modern constitutionalism, in search of possible of the financial reserve, certain that the ethics of public economy is a sine qua non condition for legal ethics.

O direito fundamental contra a autoincriminação: a análise a partir de uma teoria do Processo Penal Constitucional

The right against self-incrimination is a fundamental right that works in the criminal prosecution, and therefore deserves a study supported by the general theory of criminal procedure. The right has a vague origin, and despite the various historical accounts only arises when there is a criminal procedure structured that aims to limit the State´s duty-power to punish. The only system of criminal procedure experienced that reconciles with seal self-incrimination is the accusatory model. The inquisitorial model is based on the construction of a truth and obtaining the confession at any cost, and is therefore incompatible with the right in study. The consecration of the right arises with the importance that fundamental rights have come to occupy in the Democratic Constitutional States. In the Brazilian experience before 1988 was only possible to recognize that self-incrimination represented a procedural burden for accused persons. Despite thorough debate in the Constituent Assembly, the right remains consecrated in a textual formula that´s closer to the implementation made by the Supreme Court of the United States, known as "Miranda warnings", than the text of the Fifth Amendment to the U.S. Constitution that established originally the right against self-incrimination with a constitutional status. However, the imprecise text does not prevent the consecration of the principle as a fundamental right in Brazilian law. The right against self-incrimination is a right that should be observed in the Criminal Procedure and relates to several of his canons, such as the the presumption of not guilty, the accusatory model, the distribution of the burden of proof, and especially the right of defense. Because it a fundamental right, the prohibition of self-incrimination deserves a proper study to her constitutional nature. For the definition of protected persons is important to build a material concept of accused, which is different of the formal concept over who is denounced on the prosecution. In the objective area of protection, there are two objects of protection of the norm: the instinct of self-preservation of the subject and the ability to self-determination. Configuring essentially a evidence rule in criminal procedure, the analysis of the case should be based on standards set previously to indicate respect for the right. These standard include the right to information of the accused, the right to counsel and respect the voluntary participation. The study of violations cases, concentrated on the element of voluntariness, starting from the definition of what is or is not a coercion violative of self-determination. The right faces new challenges that deserve attention, especially the fight against terrorism and organized crime that force the development of tools, resources and technologies about proves, methods increasingly invasive and hidden, and allow the use of information not only for criminal prosecution, but also for the establishment of an intelligence strategy in the development of national and public security

Pobreza multidimensional na região nordeste: uma aplicação da Teoria dos Conjuntos Fuzzy (em 2010)

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

A relação teoria-prática do curso de formação de professores do campo na UFPA

Ce travail est une reflexion sur les pratiques éducatives dévelloppées par des élèves/professeurs dans le Cours de Formation d Éducateurs de la Campagne, réalisé dans un partenariat entre l Université Fédérale du Pará (UFPA), Le Movimento dos Trabalhadores sem Terra (MST Mouvement des travailleurs sans Terre) et le Programme National d Éducation dans la Réforme Agraire (PRONERA), en ayant comme son objectif stratégique rechercher les éléments facilitateurs de l articulation théorie-pratique dans les procédures méthodologique du Cours, fondé sur Paulo Freire. Nous approfondissons des réflexions sur la production d une proposition d Éducation de la Campagne qui, entre autres éléments, se fonde sur la Pédagogie de l Alternance, spécifiquement en ce qui concerne lês catégories Temps École et Temps Communauté. Dans le procès de recherche, nous utilisons comme ressources les documents écrits (dês projets de cours, des rapports, des journaux de la campagne document qui registre le quotidien des activités et des choses réalisées), et les discours des élèves de l Assentamento Palmares II , pendant et après lê Cours. À l analyse, nous identifions comme éléments facilitateurs de la relations théorie et pratique, le Temps École et le Temps Communauté en créant des opportunités, la rélation de ce qui se caracterise comme praxis (action-refléxion-action), en utilisant la recherche comme moyen d insertion dans la réalité et l´existence de groupe (colectif), ce qui a possibilité la participation colective aux réalisations des pratiques pédagogiques plus significatives dans le procès de Formation d Éducateurs de la Campagne, en ayant comme objectif la contribution dans la formation des sujets réalisateurs de leur histoire

Adrien-Marie Legendre (1752-1833) e suas obras em Teoria dos Números

The present thesis is an analysis of Adrien-Marie Legendre s works on Number Theory, with a certain emphasis on his 1830 edition of Theory of Numbers. The role played by these works in their historical context and their influence on the development of Number Theory was investigated. A biographic study of Legendre (1752-1833) was undertaken, in which both his personal relations and his scientific productions were related to certain historical elements of the development of both his homeland, France, and the sciences in general, during the 18th and 19th centuries This study revealed notable characteristics of his personality, as well as his attitudes toward his mathematical contemporaries, especially with regard to his seemingly incessant quarrels with Gauss about the priority of various of their scientific discoveries. This is followed by a systematic study of Lagrange s work on Number Theory, including a comparative reading of certain topics, especially that of his renowned law of quadratic reciprocity, with texts of some of his contemporaries. In this way, the dynamics of the evolution of his thought in relation to his semantics, the organization of his demonstrations and his number theoretical discoveries was delimited. Finally, the impact of Legendre s work on Number Theory on the French mathematical community of the time was investigated. This investigation revealed that he not only made substantial contributions to this branch of Mathematics, but also inspired other mathematicians to advance this science even further. This indeed is a fitting legacy for his Theory of Numbers, the first modern text on Higher Arithmetic, on which he labored half his life, producing various editions. Nevertheless, Legendre also received many posthumous honors, including having his name perpetuated on the Trocadéro face of the Eiffel Tower, which contains a list of 72 eminent scientists, and having a street and an alley in Paris named after him

Como vão se formando os professores em física e química: embates entre o ser, o ter e o fazer na formação de licenciandos da UFRN sob a perspectiva da teoria das representações sociais

The purpose of this work is to approach and understand the Social Representations (SR) (MOSCOVICI, 2003) about Physics and Chemistry from people who are major in these courses, as well as their Social Representations about teaching . We took as principle that approaching these representations it would be possible to relate their symbolic contents, in order to show how people who are following the first segments of bachelor degree courses in Physics and Chemistry become teachers, taking into account a psychosocial view. Two source of data was used during this research: Free-association Technique FA (ABRIC, 1994); and Multiple Classification Procedure (MCP) (ROAZZI, 1995). The analytical treatment of the collected data from FA was done according to the proposition of Grize, Vergés and Silem (1987 apud ABRIC, 1994, p. 66). MCP data were analyzed through MSA (Multidimensional Scalogram Analysis) and SSA (Singular Spectrum Analysis) methods associated with the Facet Theory (BILSKY, 2003). The discourses of MCP discussing groups at the moment of explanations were studied by Content Analysis as it was proposed by Bardin (1977) and Franco (2005). Indicative of an approach to the relations with knowledge (CHARLOT, 2000), the connections which aroused from the analyses showed that the group of future majors in Physics thought that this scientific field was based on a rationalist conception, influencing the idealization sense of the phenomena to be explained by Physics. Thus, Physics as school content started to require the student of the fundamental and high school to think abstractly as a cognitive skill of learning. The identifying elements observed in the relations between SR about Physics and Teaching aroused from the antagonism between future majors and their teacher, as well as from the speculation between their fundamental and high school students and themselves, mainly when they had to face the act of teaching due to the obstacles imposed by the own educational system, and by the weakness of the initial preparation. The group of future majors in Chemistry, through its discourses, showed these relations when they conceived empiricist Chemistry and said that teaching was the way of transmission of this knowledge, and didactics of Chemistry teaching was the direction to learning through pedagogic methods in order to lead the students to discoveries. The psychosocial contents which were built and showed from the symbolic relations in the studied SR achieved the relation of identity. This relation revealed identifying elements for these people, resulting from the traffic between their condition as students of Chemistry, and as teachers regarding their work, what placed the current relational contents in the teaching space, named as Knowledge changing and Adaptability . In order to study emerging questions in the discussing environment about formation and teaching professionalization, we focused the psychosocial view on this traffic and managed to observe epistemological practical and pedagogic obstacles that limited a configuration of the teaching work as a professional activity, especially from the particular conditions which led the relations of senses to Physics , Chemistry and Teaching ; and Chemistry and Physics as it was seen in this research. Generally speaking, we noted that these obstacles can denounce such obstacles concerning to the pedagogic doings which mainly impair the learning process of fundamental and high school students