There has been a lot of debate internationally about contextual effects, in terms of social and/or ability mix, and student outcomes. However, many studies have examined secondary level, but not primary education (for exceptions, see Mortimore et al., 1988; Teddlie and Stringfield, 1993), and, due to a tendency to focus on outcomes rather than processes, have been largely atheoretical in approach. Consequently, it has been difficult to determine the processes explaining school mix. This paper examines school social mix using recently-acquired data from the ADDITION study of primary school effectiveness (2010-2011). Drawing on data from a representative sample of primary schools in Ireland (N=61), the paper examines student and classroom levels and explores two key questions: (a) does school social mix in Irish primary schools impact on student achievement levels, particularly in Mathematics and Science, and (b) does teacher’s instructional role vary according to the differences in schools social mix. The results are discussed in the context of theoretical and empirical models of education effectiveness, namely, Creemers and Kyriakides’ (2008) Dynamic Model of educational effectiveness and Kyriakides and Creemers’ (2008) study of the impact of classroom-level factors on student achievement.