This article examines the effect of the ethnic composition on school performances in secondary education for Turkish students, using both cross‐national and Swiss national PISA 2009 data. At the school level our results show no effect of the proportion of natives or the proportion of coethnics and a negative association between ethnic diversity (we employ a residualized score of diversity on the proportion of migrants) and math performances. Consequently, we find no evidence for social capital advantages and an indication of barriers. Finally, we find no association between social capital variables on national or educational system level and math performance.