In 1982, pilot tests breaking with the idea of equal treatment were introduced in an attempt to do something about persistent school failure rates among the most disadvantaged pupils. These priority education zones (ZEPs) initially set up in a few regions were strengthened and extended in 1989 and 1990 and the measure has been regularly renewed since. It encourages the establishments concerned to develop educational projects and local partnerships by granting them additional resources (appropriations, jobs, hours of teaching, etc.). The aim is to improve educational performance by stimulating new projects. A reduction in class sizes is also coming to be seen as a tool. The changes that ZEP status has brought about for these establishments are evaluated using administrative data on the schools. The drop in class size is very gradual. In the early 1990s, expenditure in ZEP establishments was far from negligible, but was made up mainly of weighting allowances paid to staff. Moreover, the schools concerned posted a reduction in pupil numbers, greater social homogeneity and an increase in the proportion of young teachers. Evaluating the impact of the ZEP policy entails more than just a comparison of the success rates of pupils in ZEP establishments with others based on observed characteristics. The priority zones are chosen precisely because they concern a more disadvantaged population and poorer educational performance than elsewhere. A specific context effect, existing before the implementation of the priority zone policy, is therefore probable and may not be totally taken into account econometrically based on the variables used. A direct estimation would therefore merge the impact sought with the context effect. This ZEP endogeneity needs to be addressed and is here by the differences-of-differences procedure, which neutralises the context effect in place before the introduction of the priority zones, on the assumption that this effect remains sta