This article estimates the effects of school expenditure on school performance at Key Stage 4 in England, over the period 2003-07 during which real per pupil expenditure increased rapidly. It adds to previous investigations by using dynamic panel analysis to: exploit time series data on individual schools that only recently has become available; adjust for the potential endogeneity not only of expenditure but also of other determinants of performance; and differentiate the short-run and the (higher) long-run attainment effects of spending changes. Consistent with other recent work, the article reports a generally significant but small effect of expenditure on school performance, but it also finds that the effect varies between specialist and non-specialist schools, with the effect on the latter being larger. Further, the article identifies significant dynamics in the school improvement process, quantifies the long-run effect of expenditure changes, suggests that spending effects increase with socio-economic disadvantage, and quantifies absence effects. (Contains 2 notes, 5 tables, and 6 figures.)