Abstract : This Thesis addresses the issue of peer-effects in the context of school. From analysis of a large database produced by a Chilean national study (SIMCE 2004), this work investigates the mechanisms through which pupils with different levels of scholastic, human and cultural capital influence each other. These influences seem present for a diverse range of school outcomes, including academic achievement. Drawing on the literature produced by different disciplinary approaches —sociology, economics, social psychology and education— the study focuses on ways of identifying and measuring peer-effects. The presence of subjective dimensions capable of reflecting, in part, the school experience of pupils is also taken into consideration. Beside, those dimensions are connected both with peers presence and peer interactions. In addition, the thesis re-examines the existing literature on the Chilean school system, including its social and academic segregation and its relationship with the voucher system. Within this framework, three main points of interrogation organize this work. First, whether study practices which involve peer assistance have a clear impact on standardized school test scores. Second, the likely presence of influences, —in the form of “capitals transfers”— between pupils with different backgrounds who practice peer assistance. Finally, the visible relationships between peer assistance and feelings of well-being at school or academic self-concept, but also, between the latter and school achievement. A sequence of analysis was undertaken out to provide robust foundations for possible answers to those questions. Among other things, different sets of hierarchical and quantile regression analysis were conducted in four school subjects. The main research findings show, on the one hand, that peer assistance between pupils is fairly widespread in the school context (between 22% and 41% in average) but its prevalence varies according to the subjects and the directionality of the assistance. Further, those peer assistance is significantly related at school achievement. In all subjects and at similar conditions, poor achievers benefit from help by their classmates. At the same time, those that help their classmates always showed a strong academic profile related to the higher and greater gains in test scores. In the other hand, we find that pupils with more cultural capital, all other things held constant, are more likely to report help other pupils. Finally, this research confirms that the relationship peer assistance and feelings of school well-being and academic self-concept is large and significant. Several secondary findings were also produced and discussed, including confirmation, for the first time in Chilean case, of the hypothesis associated with the paradigm BFLPE (Marsh, 1987). Those results are discussed with regard for their probable consequences in terms of educational policy, particularly in systems with high academic and social segregation.