The authors use administrative data from three large urban school districts to describe student sorting within schools. Students are linked to each of their teachers and students' classmates are identified. There are differences in the average achievement levels, racial composition, and socioeconomic composition of classrooms within schools. This sorting occurs even in self-contained elementary school classrooms and is much larger than would be expected were students assigned to classrooms randomly. Much of the racial and socioeconomic sorting is accounted for by differences in achievement, particularly at the high school level. Classrooms with the most low-achieving, minority, and poor students are more likely to have novice teachers. Sorting students by achievement level exposes minority and poor students to lower quality teachers and less resourced classmates.