How do peer groups influence academic self-concept formation? We evaluate developmental issues in the big-fish-little-pond effect (BFLPE; negative effects of class-average achievement on math self-concept [MSC]) and its generalizability to peer-group-average achievement (1,017 primary-school students tested in Years 4 and 6, 46 classes, 130 peer groups). The effects of peer-group-average and class-average achievement on MSC were both negative when we considered these two contextual effects separately. However, the effect of peer-group-average became nonsignificant in models with both contextual effects; the negative effect of class-average achievement was relatively unaffected. Results for peer-group-average achievement contradict predictions based on local dominance theory (that the BFLPE should be more negative for peer-group-average achievement than the more local frame of reference, a contrast effect) and social comparison choice studies (that peer-group-average achievement effects should be positive, an assimilation effect). Unsurprisingly, we found BFLPEs based on class-average achievement and gender differences favoring boys in both Years 4 and 6. However, consistent with theories of the cognitive development of social comparison and gender socialization/intensification processes, we also found negative effects of class-average and gender differences favoring boys on change in MSC (MSC in Year 6 controlling for MSC in Year 4) over this critical late-childhood period. Our results support the robustness of the BFLPE based on class-average achievement and developmental processes underpinning it, but do not support the posited effects of peer-group-average achievement.