Everywhere school reproduces social inequalities as it is more favourable to already advantaged pupils. However this « law » is too general to explain the large variations in the amplitude of this reproduction revealed by international comparisons. Relying upon these studies, the paper shows first that these variations cannot be explained directly by the amplitude of social inequalities, so that, to explain them, it is necessary to investigate two main factors. The first one concerns the organization of the school systems. It proves that it may widen or mitigate the impact of pupils’ social background on their academic achievement. The second one concerns the influence of diplomas on access to various social positions. The paper shows that the more diplomas play a deciding role, the more school inequalities are pronounced and the more the reproduction of social inequalities is rigid. Finally, it is the function attributed to school by the various societies that determines the amplitude of the reproduction. However, this study achieved on a sample of countries stands at an aggregate level, and some qualitative complementary studies would be very useful to better understand how social reproduction operates.