It is now well known that adolescence is frequently marked by a decline in students' achievement motivation, which in turn is often associated with a decline in individual school self-concept. Less is known about the mediating role of socio-motivational support in the association between individual school self-concept and achievement motivation. The current study examined the interplay of individual school self-concept, socio-motivational support and achievement motivation in a large sample of seventh and eighth grade students (N?=?1,088; M [subscript Age]?=?13.7) in secondary schools in Brandenburg, Germany. Structural equation modeling was used to test the associations between individual school self-concept, socio-motivational support, and achievement motivation. The results showed that the teacher-student relationship as well as "teachers as positive motivators" mediated the association between individual school self-concept and achievement motivation. In contrast, neither "peers as positive motivators" nor the student-student relationship mediated this association. These results support the notion that maintaining a positive teacher-student relationship as well as encouraging teachers in the role of positive motivators could be an effective starting point for prevention and intervention programs aimed at offsetting the decline in individual school self-concept and achievement motivation during adolescence.