This study investigates whether improving student mobility leads to greater sorting of students between schools and classes. I isolate an exogenous change in student mobility using the two-stage design of the Polish comprehensive education system and differences in school density across geographic areas. I construct a novel measure of student homogeneity based on Raven’s Progressive Matrix test score. One finding is that higher mobility leads to greater sorting of students between schools. Another, more novel, result shows that mobility also leads to higher sorting within a school (across classes). I provide suggestive evidence that demand for peer quality among students motivates school principals to create selective tracks within comprehensive schools.