Investigating the effects of class average achievement: attending a high-achieving class is neither beneficial for student achievement nor for academic self-concept

• We test the effects of class-average achievement and teaching quality on students. • Controlling for pre-selection effects strengthened the Big-Fish-little-pond-effect. • Controlling for pre-selection effects weakened the peer spillover effect. • Teacher enthusiasm and classroom management had little added value for achievement. • High class-average achievement does not benefit students’ achievement or self-concept. Research has shown positive effects of group-average achievement on individual achievement (peer spillover effect) but negative effects on academic self-concept (big-fish-little-pond effect; BFLPE), which is positively related to achievement. We examine these apparently paradoxical class composition effects in a longitudinal sample of more than 2,000 German students followed over two school years in a tracked school system. Using multilevel models that control for school track, prior ability, socioeconomic status, gender, and two indicators of teaching quality (teacher enthusiasm and classroom management), as well as measurement and sampling error, we test the robustness of both effects. Results show that the peer spillover effect is largely explained by pre-existing differences, such as prior ability. In contrast, the BFLPE became more negative after including covariates and remained so when controlling for teaching quality. The inclusion of teacher enthusiasm and classroom management only marginally changed either effect, indicating that, in this dataset, these two core instructional variables explained little of the variance in achievement or self‐concept beyond pre‐selection and peer composition effects. By distinguishing between compositional effects and instructional influences, our study contributes to a clearer understanding of how peer and classroom factors interact. Implications for future research include examining a broader range of teaching quality dimensions and exploring the conditions under which class composition may positively affect both achievement and self-beliefs.