This study examines the relation between the proportion of co-ethnics in school and adolescents’ problem behaviour in school (e.g. skipping class and arguing with teachers) and whether friendship patterns are underlying this relationship. We use data from the Children of Immigrants Longitudinal Survey in Four European Countries on ±16,000 students in England, Germany, The Netherlands, and Sweden and find that children display less problem behaviour when the proportion of co-ethnics in school is higher. This relationship is mediated by the characteristics of the friends that students have: the proportion of co-ethnics in school positively relates to students’ proportion of in-school friends and co-ethnic friends in class, which are in turn negatively associated with problem behaviour in school. The strength and significance of these paths depend on students’ ethnicity and country of residence. Implications of this study are discussed in the conclusion.