The work emphasizes that the roots of inequality find fertile breeding grounds on the educational systems and focuses on classroom aiming to understand possible sources of inequality among mates because it is an environment where students interact, sharing much time together. In detail, I investigate how hierarchies, networks of friends, and classroom peers influence students’ motivations, aspirations, academic competences, behaviors, and educational choices. In chapter I of this work, I outline a theoretical framework arguing that classroom inequality is a result of varying characteristics of the interacting actors such as their gender, age, ethnic origin, socioeconomic background as well as academic competencies. The classroom sorting of students with specific characteristics broadly depends on formal and informal institutional rules. To shed light on these patterns of educational systems, I rely on three distinct concepts, such as inequality, diversity, and sorting. In chapter II of this work, I test whether teachers’ grading is an inequality-enhancing factor in Italy. Previous contributions suggest that teacher’s grading is biased by preferences and stereotypes. My idea is that teachers’ grading standards might produce a hierarchy among students, even among equally able students. This hierarchy, in turn, could have a pervasive influence on students’ perception of their own competencies, thereby influencing their academic achievement, motivation, and self-stigma. In chapter III, I investigate the extent to which extent smoking and drinking friends lead to emulate the same behavior in a critical age like the adolescence. Unhealthy habits dramatically affect life expectancy, above all, when rooted in the early stage of individual development. In addition, I analyze if non-reciprocal friendship matters more or not as a driver of the behavior emulation because adolescents desire to be accepted. In final chapter IV, I test to what extent the presence of students with a migration background affects several outcomes in classrooms, including students’ attitudes and anti-social behavior. Italy is dealing with a dramatic increase of immigrant students since the late ‘80s, but a series of data suggest that the school is not well equipped for this challenge. Overall, the thesis aims to contribute to important theoretical debates in the sociology and economics of education, such as the role of relative positions in the social environment (chapter II), peer effects in critical developmental stages (chapter III), and the social integration in heterogeneous contexts (chapter IV). However, it aims also to inform policymakers on possible side effects of current widespread educational practices such as grading on a curve (chapter II), the actual role of peers in the spreading of unhealthy behaviors among adolescents (chapter III), and the need of imposing interventions devoted to optimizing classrooms compositions (chapter IV).