As levels of immigration and ethnic diversity continue to rise in most Western societies, the social demography of schools is changing rapidly. Although schools represent a prominent developmental context, relatively little is known about the extent to which the racial/ethnic composition of schools influences mental health outcomes in students. The objective of the present study is to examine the association between immigrant and racial/ethnic congruence in school—the numerical representation of a student’s immigrant generational status and race/ethnicity in the student body—and levels of emotional and behavioral problems. This study also examines the extent to which the association between congruence and emotional-behavioral problems differs across racial/ethnic immigrant sub-groups and is accounted for by individual perceptions of school belonging. Data come from the in-school survey of the Longitudinal Study of Adolescent Health (Add Health) conducted in the United States. The sample is nationally representative, and includes 128 schools and 77,150 adolescents in grades 7–12 (50 % female, M age = 14.9 years, SD = 1.78). After controlling for school and family socio-demographic characteristics, immigrant and racial/ethnic congruence in school exhibited a negative association with emotional and behavioral problems for most sub-groups examined. School belonging was associated negatively with emotional and behavioral problems, and partially accounted for the effects linked to congruence in schools. The immigrant and racial/ethnic composition of schools and perceptions of belonging have strong links with emotional and behavioral problems and may represent important targets for intervention.