Whether school composition makes a difference for student outcomes has been the subject of much controversy. This article draws on "Growing Up in Ireland" data, a rich source of information on nine-year-old children along with the characteristics of their school and classroom teacher. In contrast to many studies which rely on a single measure of social background, the use of a range of measures (including social class, parental education and income) allows us to provide precise estimates of differences in performance between schools designated as disadvantaged and other schools. The analyses also provide new insights into the processes underlying such differences. Only the most disadvantaged schools, those in Urban Band 1, are found to have a contextual effect for both reading and mathematics. This achievement gap is found to reflect differences in teacher experience and turnover, the concentration of additional learning needs, absenteeism levels and children's engagement in school.