Social comparisons with peers are important sources of self development during adolescence. Many previous studies showed that to form their academic self-concepts (ASC), students contrast their achievement with the average of their class or school (the Big-Fish-Little-Pond Effect, BFLPE). Based on social comparison theory, however, we would expect some peers to be more likely social comparison targets because they are more visible or students perceive them as similar to themselves. In this study, we used sociometric data to analyze which peers play the most important role for social comparison effects on ASC. We examined how the average achievement of friends, study partners, popular peers as well as same-gender and same-ethnic peers affect the general ASC and how these effects compare to the effect of classroom average achievement. The study was based on a German longitudinal sample of 2,438 students from 117 school classes that were followed from Grade 9 to 10. As a novelty in ASC research, we applied methods for longitudinal social network analysis. . Our results do not confirm substantial incremental effects of specific peers, while class average achievement showed a stable negative effect (the BFLPE). In addition, we could provide evidence for social selection effects based on ASC. Controlling for endogenous network processes and similarity in achievement, gender, socioeconomic background, and ethnicity, students were more likely to select peers with similar ASCs as friends. We conclude that classrooms provide a specific setting that imposes social comparisons with the “generalized peer” rather than with specific subgroups of peers.