Studies conducted in primary schools give evidence, that classroom composition concerning antisocial behaviour might influence individual behavioural pathways. In this cross-sectional study, it is tested, if such findings might also apply to students in lower secondary schools (7th- to 9th-grades). In a sample of 493 students, it was found, that the level of antisocial behaviour within the personal peer environment (mean of all classmate`s behaviour without individual value) significantly predicts individual aggressive-oppositional and delinquent-criminal behaviours. This remains true, even when controlling for sex, antisocial attitudes, academic track and grade. Interaction effects show a special impact of the classroom peer environment on the behaviour of boys and students with positive attitudes towards delinquent-criminal behaviour. The results of this study indicate a significant relationship between classmates` and individual behaviour. They are discussed in the light of research on class composition and the question of adequate educational placements for students with antisocial behaviours.