Using PISA data for 16 Western OECD countries having comprehensive school systems, we explore the conditions under which the socioeconomic composition of schools affects educational efficiency and equality, to a greater or lesser extent. First, a multilevel analysis is applied to examine and compare the effect of school socioeconomic composition on students' outcomes across countries and comprehensive models. Second, a simulation exercise shows the variations in the efficiency and equality levels that would result in two distinct hypothetical school scenarios in each country--a segregated scenario and a nonsegregated scenario. We find that a hypothetical reduction in school segregation would positively affect educational equality in all of the countries considered, but the impact on levels of educational efficiency in individual countries varies with the structure of comprehensive schooling.