Constrict theory states that, in the short run, ethnic diversity in any context lowers both the quantity and the quality of interpersonal contacts. We test this theory in the school context, expecting that ethnic school diversity yields fewer and lower quality friendships. Moreover, we investigate whether the associations hold when controlling for the school's socioeconomic situation, and whether the relations between ethnic school diversity and the social outcomes differ between natives and immigrants. Multilevel analyses on data from the Flemish Educational Assessment, consisting of 10,546 natives and 1,324 immigrants in 85 secondary schools, show that ethnic diversity yields fewer friendships and a lower attachment to friends. However, this appears to be due to the schools’ socioeconomic composition. For immigrants, moreover, a higher ethnic diversity yields more friendships and a higher attachment to friends. Implications for theory and practice are discussed.