This paper examines assumptions which underpin the promotion of a quasi-market in the English secondary school system in light of research findings on pupil mobility--that is, children joining and leaving schools at non-standard times. It draws principally on research funded by the Nuffield Foundation and reports findings of a study of pupil mobility in secondary schools in three urban education authorities. It shows that school options for most mobile pupils are limited and their circumstances can impede considered choice. The operation of the quasi-market is found to produce schools with high levels both of pupil mobility and of deprivation. The assumption that choice will lead to better schools because the undersubscribed will be under pressure to improve is found to be flawed, since the supply of children is not necessarily inelastic and pupil mobility can place huge demands on school staff which divert time and resources from school improvement. (Contains 3 tables.)