While school self-concept is an important facilitator of a student's school engagement, previous studies rarely investigated whether it may also explain the change in students' school engagement during secondary school. Moreover, as social relations play an increasingly important role in adolescence, the current research distinguishes between the social and individual school self-concepts of a student. Whereas individual school self-concept uses the perception of a student's own ability in the past in order to estimate perceived current ability, social school self-concept refers to the comparison of a student's own perceived current ability with the current perceived abilities of others. We examined the role of students' individual and social school self-concepts in the development of behavioral and emotional school engagement during the period from grade 8 to grade 9. The sample consisted of 1088 German adolescents at the first measurement time (M[subscript age] = 13.70, SD = 0.53; 53.9% girls). The findings suggested a significant decline in both emotional and behavioral school engagement over the span of 1.5 years. In addition, social--but not individual--school self-concept was associated with the change in both dimensions of school engagement over time, such as it may intensify a student's decline in school engagement levels. This might be due to the fact that students with a high social school self-concept tend to increasingly emphasize competition and comparison and strive for high grades, which lowers students' school participation and identification in the long term.