This article shows that an ethnically diverse student population leads to blurred ethnic and racial boundaries in high schools. Still, students in New York distinguish themselves much more along ethnic and racial lines than do London students. The evidence presented suggests that, in addition to national-level differences, traditional British school structure, which provides continuity of peers through the form class as well as time for socializing, leads to less emphasis on ethnic and racial boundaries than in the anomic structure of traditional urban American public high schools. It follows that, to promote ethnic and racial integration among teens, schools should not only serve integrated student bodies but also should maintain structures that present opportunities for students to bridge racial and ethnic boundaries. The study employs ethnographic data from schools in New York and London and 120 in-depth interviews.