To advance social integration, policy makers strive for the educational desegregation of immigrant students in Flemish schools. Given the lack of empirical research supporting this policy, this article examines the association between the ethnic composition of schools and native and immigrant students' interethnic friendships, social participation, and sense of belonging in school. Blau's structural theory offers the theoretical rationale for these associations and the coherence of the three indicators. Multilevel analyses of data from a 2004-05 survey of 11,872 students, 1,324 of whom were immigrant students, in a sample of 85 Flemish secondary schools demonstrate that school ethnic composition is associated with interethnic friendships and social participation for native students, but not for immigrant students, whereas socioeconomic status is decisive for immigrant students' interethnic friendships. Neither immigrants' nor natives' sense of belonging in school is associated with ethnic composition. Hence, while the findings do not provide support for either school segregation or desegregation policies aimed at improving the social integration of immigrant students, mixing schools appears to have a positive influence on the social integration of Flemish youths. The consequences of these findings for future research and social policy are discussed in the conclusions.