The ethnic classroom composition and classmates' ethnic attitudes can affect how students experience their classroom social environment (CSE). Following the imbalance of power thesis and prior research on ethnic attitudes, this cross-sectional study examined if ethnic classroom composition (i.e., proportion of in-group and Herfindahl Index) and classmates' explicitly and implicitly measured ethnic attitudes predicted secondary school students' (M[subscript age] = 13.31 years; 58.1% female) classroom belonging, popularity and likability, classroom cohesion and conflict in mixed classes in the Netherlands. Differences between non-ethnic Dutch (n = 248) versus ethnic Dutch students (n = 141) were examined as well. Ethnic Dutch students report an overall more negative CSE than their non-ethnic Dutch classmates. Multilevel analyses indicated that a higher proportion of in-group peers affected non-ethnic Dutch students' popularity and likability negatively. Moreover, classmates' explicitly measured ethnic attitudes were predictive of student popularity while classmates' implicitly measured ethnic attitudes were predictive of student likability. Finally, classmates implicitly measured ethnic attitudes moderated the effect of proportion in-group peers on students' shared experience of classroom belonging. These results show that promoting classroom diversity is not enough to create a positive CSE for all students. Classmates' ethnic attitudes are also important to consider.