The objective of this study was to identify gender issues in the academic performance of boys and girls during physics classes in a laboratory. The methodology adopted was the observation and interactions of pupils during eight classroom events. The interactions were recorded and events were informally discussed with the teacher. The school visited is a Federal mixed school, located in the city of Belo Horizonte, a large urban and industrialised area of Brazil. Boys and girls differed significantly in their interaction at school. Boys seemed determined to impose their will and to defend their masculine identity at any cost, even at the cost of academic failure. Girls seemed to seek the approval of the teacher, needed more affirmation and usually requested the teacher's attention from their desks. Boys' and girls' behaviour in the school expresses the different socialisation models given to them by society. It is suggested here that schools are social institutions in which values and attitudes that reinforce social division predominate. Schools validate ideas about "boys' nature" (masculine identity) and "girls' nature" (feminine identity), "positioning" boys and girls in different social spaces. Schools and teachers must intervene in classrooms to guarantee a better academic involvement of boys and girls. (Contains 3 tables and 5 notes.)