BackgroundAcademic self-concept (ASC) is a key predictor of learning behaviors and educational outcomes. In adolescence, the evaluation of academic abilities is mainly shaped by the social environment and comparisons with various reference groups. The effect of making social comparisons with the academic achievement of a reference group is known as the big-fish-little-pond effect (BFLPE). AimsBased on social comparison theory and the local dominance effect (LDE), the present study aimed to investigate a pivotal adolescent reference group beside the classroom: the clique. We investigated to what extent students’ social comparison with the mathematics achievement of clique members was related to the ASC of individual students more than general classroom comparisons. SampleThe sample comprised 743 German secondary students in sixth and eighth grade (93 cliques in 40 classrooms). MethodsWe estimated a three-level structural equation model using data from a two-wave longitudinal study in one school year. The change in students’ ASC was determined using latent change modeling. ResultsThe average academic achievement of the clique negatively predicted ASC development. Classroom academic achievement had no effect. ConclusionsThe results underline the importance of informal peer groups like cliques as important reference groups for social comparisons and ASC development during adolescence.