We analyze whether non-native speakers in the classroom affect students' educational achievement and social integration. In contrast to previous studies, which mainly examine the effect of the share of immigrant pupils, we focus on language heterogeneity by using a novel measure of the degree of linguistic diversity in the classroom. Conditional on the concentration of non-native speakers in the class, the degree of linguistic diversity has no adverse effect on students' language and math skills, but worsens the social integration of non-native speakers. We demonstrate the robustness of these findings in a variety of sensitivity checks.