This paper studies how high and low performance depends on the students’ family environment, in the OECD countries, using the data on mathematical competencies in PISA 2012. In particular, we compare the results of those students coming from disadvantaged and advantaged families, identified with those in the first and fourth quartiles of the distribution of the index of Economic and Socio-Cultural Status. The results show that the differences in the educational systems between the OECD countries are larger than usually recognised. They also make it clear that socio-economic conditions strongly determine the distribution of high and low performers, with substantial differences between the countries.