In this study the authors start from the observed fact that equality of opportunities of educational achievement is higher in integrated school systems than in differentiated school systems. In other words, in integrated school systems, a pupil's school achievement depends less than elsewhere on the social and cultural resources of his or her family. However, before concluding that school structure has a significant influence on inequalities at school, it is important to distinguish between the influence of the socio-economic context underlying each school system, and the specific influence of the structure or organisation of the school system itself. The fact is that not only do the most egalitarian countries with regard to schooling have in common an integrated structure, but also these schools are set within the context of countries which are more egalitarian in other ways, particularly with regard to income distribution. To distinguish between the influence of this social environment and that of school structure, the authors offer three analyses based on a comparative analysis of international databases measuring educational achievement. The results of these three analyses lend credence to the hypothesis that the structure of the school system has a specific effect on the extent of inequalities. (Contains 3 tables, 2 graphs, and 5 notes.)