This thesis addresses the issue of educational inequality of opportunity by exploring how countries and schools differentiate their students for instruction during lower secondary schooling. Using a combination of comparative work examining twenty four countries using the data on student achievement among 15 year olds from the Programme for International Student Assessment (PISA) 2006 and analyses with longitudinal case-study data from Ireland, this study builds upon existing knowledge in the area of stratification of educational opportunity. The methodology lends itself to a detailed investigation of the differentiating practices within countries and within schools and their impact on student achievement. In particular, this thesis focuses on the relationship between socioeconomic background, socioeconomic composition of the school and how students are sorted for instruction. In an attempt to contribute to the debate surrounding tracking practices the thesis explores how school composition effects vary depending on how a country sorts its students for instruction, the relationship between tracking and school composition and the role that curricular differentiation plays in explaining differential achievement. The findings highlight that the known effect of how countries sort their students for instruction on the inequality of educational achievement appears to be mediated through school composition effects. The results show that differentiation at the country level accentuates compositional effects, i.e. school composition matters more in highly differentiated education systems. However, the pattern is less extreme for high socioeconomic status students; that is highly differentiated systems do less to magnify compositional effects for high socioeconomic status students as compared to low socioeconomic status students. In Ireland, schools that track their students are achieving less than their counterparts in non-tracked schools which is associated with the lower social class composition of the school. Furthermore, there is inequality of educational opportunity within tracked schools. Those from lower social backgrounds do significantly worse in tracked schools compared to their peers. Access the curriculum is more differentiated in tracked schools with those in the top track gaining the best access to the higher levels of curricula. The findings in this thesis illustrate the payoff between offering an achievement advantage to a minority, or maximising achievement among the greatest number. Differentiating practices maximise the achievement advantage for the few who are high socioeconomic status students in high socioeconomic composition schools. Schools that track maximise the achievement advantage for the few who enter the school with higher ability levels by giving them better access to the curriculum which widens the gap in achievement between track placements. Comprehensive education systems and schools that do not track their students keep the channels of movement between courses of study open (Turner, 1960) and in doing so appear to maximise the achievement of a greater number with a small loss of achievement for the minority at the top. By exploring the role of differentiating practices within countries and within schools, this dissertation contributes to a greater understanding of how social class status and school-composition effects vary depending on how students are sorted for instruction.