Research has shown that participation in extracurricular activities has a positive effect on adolescents' social behaviour and academic performance; however, the reciprocal influence of extracurricular activities and leisure on the development of adolescents' academic performance and social behaviour is unclear. In our study, we investigate the effect of school based and out-of-school leisure activities on adolescent's social and scholastic development. We also explore how students' gender, socioeconomic status and ethnicity influence their choice of, and engagement in activities. A sample of 5278 students (females 50%; migrant background 26%) filled in questionnaires in grade 5 and 7 and provided personal background information as well as evidence of their engagement in extracurricular and leisure activities, their social behaviour and academic performance. Using latent class analysis, we distinguished five identity-related patterns of engagement in extracurricular and leisure activities among 5th-graders. We found a connection between adolescents' gender, socioeconomic status, and ethnicity and their choice of, and engagement in, extracurricular and leisure activities, social behaviour and grades. We also identified a link between adolescents' patterns of engagement in extracurricular and leisure activities and developmental tendencies in their social behaviour and scholastic achievement between grades 5 and 7.