This open access book uses multiple IEA Assessments to examine the relationship between socioeconomic segregation between classrooms and student outcomes. By examining Socioeconomic status (SES) segregation between classrooms as well as between schools, it produces a more accurate estimate of student sorting. Further, this study examines the differential impact of student sorting across subject areas and grades in order to explore whether school structure’s relationship to educational inequality exhibits content and longitudinal heterogeneity. This study employs time series, fixed-effect, random-effects, and synthetic-cohort methods to comprehensively investigate the robustness of the relationship between SES segregation and achievement inequalities. This project makes an important contribution to researchers’ understanding of student sorting’s impact using a comparative lens, while also providing important information to policymakers on the role of schools in mediating social inequalities.