This paper aims at assessing the prevalence and learning consequences of ability grouping in French lower secondary schools (collèges). The paper shows that ability grouping is in fact relatively widespread in France and has a strong local connotation. In terms of pupil learning, the paper stresses that students tend to perform all the more better the better the class they attend. However an heterogeneous context proves to be more favourable (for low achievers, an homogeneous class is the worst case). The results also stress the existence of a potential conflict of interest between the policy-maker and the actors of the system.