While the share of non-native students in a class is supposed to have a non positive effect on school achievement, little is said about the heterogeneity of the ethnic minority make-up. Ethnic diversity can stimulate the creativity of students, can push them to be proficient in the instructional language, can reduce the scope of ethnic identification with all its possible drawbacks, but it may also worsen social interactions among pupils and make the job of teachers more difficult. I exploit the within school cohort variation in ethnic diversity of a rich data-set about primary education in the Netherlands to investigate whether ethnic diversity matters for school achievement, for whom it matters and which can be the mechanisms it may generate. I find that ethnic diversity has a positive impact on the test scores of minority students, in particular for language skills. I also find some evidence of a negative relationship between ethnic diversity and school social interaction among pupils.