This article describes the social and economic "segregation" of students between schools in England, and the likely causes of its levels and changes over time. It involves a re-analysis of the intakes to all schools in England 1989-2011, and shows how strongly clustered the students are in particular schools. The pattern for primary-age schools is the same as for secondary-age schools. However, each indicator of potential disadvantage--poverty, learning difficulty, first language, and ethnicity--has its own level and pattern of change over time. This suggests that there is not just one process of segregation. The implications for any state wanting a fair and mixed national school system are spelled out. (Contains 3 figures.)