Classrooms with a high proportion of students who share the same minority language …

„In classrooms with a high proportion of students who share the same minority language, it is likely that they will use that language with each other rather than the language of instruction, for example, during breaks. Students who speak a minority language with peers inevitably spend less time using the language of instruction, which should impair their acquisition of the language of instruction and, consequently, their educational achievement.“
Julian Seuring u. a., Ethnic Classroom Composition and Minority Language Use among Classmates: Do Peers Matter for Students’ Language Achievement? In: European Sociological Review, 2020, Vol. 36, No. 6, S. 923.