It is well known that teachers frequently observe a decline in the scholastic motivation of many adolescent students, which in turn is often associated with a decline in students' individual school self-concept. In contrast, less is known about the association between students' individual school self-concept and school engagement, or the related concepts of school belonging and helplessness in school. The current study explored social relationships as mediators in the association between individual school self-concept and adolescents' school engagement, belonging to school and helplessness in school in a large sample of 7th and 8th grade students (N?=?1088; M[subscript Age]?=?13.7) in secondary schools in Brandenburg, Germany. The results of two structural equation models showed that the teacher--student relationship mediated the association between individual school self-concept and school engagement, belonging to school and helplessness in school, respectively. In contrast, the student--student relationship did not mediate any of these associations.