This thesis has the objective of investigating the social construction of sex education in a public school with a Nucleus of Adolescent Multipliers (NAM). An ethnographic research was developed in a school in Rio de Janeiro, Brazil, between August, 2002, and June, 2003. During this period, NAM`s meetings, classes of various subjects, and, specially, the classes of Sciences were observed, as well as meetings, breaks, and other activities. Students and teachers were interviewed. The current intense debate about the sexuality of adolescents is caused by the fact that sexuality is an important focus of political investment and of governance technology, and, the school reveals itself as a privileged space for the development of biopower. This thesis demonstrates in what ways education is imbricated in this problematic and how it relates to other areas of knowledge, as biology, medicine, demography and psychology, in order to manage the adolescent sexuality. Sex education has been taught at schools mainly by Science teachers based on the theme of human reproduction, and, in some schools, in the framework of NAM. The school offers the students a knowledge that pretends to be scientific, and, therefore, true about sexuality, which is used by them as a criterion of truth to judge their previous knowledge about this theme. In this perspective, the human body is conceived as an organism and each organ is studied having as main focus its reproduction function. Thus, the woman`s body receives greater emphasis than the man`s body. Contradictorily, when sex education is developed on the basis of the reproduction theme, precisely this ends up being emphasized, in spite the fact that it is exactly its occurrence among adolescents that the various educational policies intend to avoid. The school interventions try to develop among adolescents a sense of responsibility around the sexual relations, aiming to change or to adequate the mechanisms that structure patterns of preventive behavior. With this objective, the school, besides fostering the use of preservatives for the practice of safe sex, ends up recommending a specific model of relationship, in which sex relations should occur. In a similar way, pregnancy appears as an inadequate experience for this period of life.