International audienceThe marked economic and social differences characteristic of the Paris region are reflected in marked intra-regional academic inequalities. These inequalities, created mostly by factors outside the education system, greatly constrain the capacities of school principals to improve social and academic diversity in their schools.This IPP Note shows that on top of the structural constraints, schools themselves limit even further the social and academic mix within their own walls by their ways of composing classes, which reinforce students’ local experiences of segregation. This influence is highly significant: within a municipality, segregation among classes in a middle school or high school is comparable to the segregatio...