Despite extensive research on multiracial youth in recent years, to date, no empirical studies have analyzed how racial context may affect biracial adolescents' sense of belonging in a social institution beyond families. In this study, we examine how the racial makeup of the student body affects self-identified biracial adolescents' school attachment. Using data from the National Longitudinal Study of Adolescent Health, we find that the proportions of white or black students in school significantly affect the school attachment of Hispanic/black, Asian/black, and American Indian/black biracial adolescents, but school racial composition in general has little influence on biracial adolescents with a partial-white identification (i.e., black/white, Hispanic/white, Asian/white, and American Indian/white). Our analyses also show that on average, students of most biracial groups display lower school attachment than their corresponding monoracial groups, but the differences from the monoracial groups with the lower school attachment are generally small. We discuss the implications of our findings for biracial adolescents' perceived racial boundaries and contemporary American race relations.