The present study aims to investigate how school segregation, as well as the (in)congruence between the school and individual SES, can explain the variation in student achievement. Additionally, it examines the role of instructional leadership in mitigating this association. Using international large-scale assessments (PISA-TALIS link data) from seven countries–Australia, Colombia, the Czech Republic, Denmark, Georgia, Malta, and Turkey–we applied several multilevel polynomial regressions with response surface analyses. The results showed that both individual SES and school segregation have a profound impact on student achievement, with varying results across countries. Second, we found differential school composition effects, with the school composition effect strongest for low SES students in high SES schools. Third, our results do not support congruence theory, but they do somewhat favor (in)congruence theory. Finally, strong leadership magnifies benefits for low-SES students in high-SES schools and for all students at low-SES schools. Implications for policy, practice, and further research are discussed.