There are large social class inequalities in educational achievement in the UK. This paper quantifies the contribution of one mechanism to the production of these inequalities: social class differences in school "effectiveness," where "effectiveness" refers to a school's impact on pupils' educational achievement (relative to other schools). It builds on the small number of existing studies, whilst overcoming a number of their limitations. It estimates the effectiveness of the (state) schools attended between ages 7/8 and 10/11 by a cohort of children born in the 1990s in Avon, a former county of England, and then compares the effectiveness of the schools attended by children from different social classes. It finds that "higher" social class children attend more effective schools, on average, and that these social class differences in average school effectiveness between ages 7/8 and 10/11 account for 7% of social class differences in average educational achievement age 10/11. This is not a causal analysis, however.