Which factors help young adults choose educational pathways leading to higher educational attainment? Academic self-concept (ASC), achievement, and socioeconomic background have all been found to be important predictors of postsecondary educational choices and success. Although research has shown that student composition in secondary school may affect some of these predictors, only a few studies (mostly from the United States) have investigated the effects of student composition on postsecondary educational outcomes. The results showed that students with similar individual achievement had lower postsecondary educational attainment if they graduated from secondary schools with higher mean achievement. It has been proposed that social comparison processes explain this negative context effect (big-fish-little-pond effect [BFLPE]). In contrast, students with the same individual socioeconomic status (SES) had higher postsecondary educational attainment if they graduated from secondary schools with a higher mean SES. In the present study, we investigated the effects of achievement-related and socioeconomic student composition on subsequent educational outcomes using data from a longitudinal study in Germany. Contrary to previous studies, our results showed that student composition had little relevance for later educational pathways. There was a small, long-lasting, indirect BFLPE of achievement-related composition in secondary school on postsecondary educational outcomes through students' ASC. Furthermore, individual SES was strongly related to postsecondary educational outcomes, whereas being in an academic-track school with a higher or lower mean SES was not relevant for postsecondary educational pathways.