The 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 segregation seen among different schools in the area. Equally surprisingly perhaps, chance explains the largest portion of internal segregation: 84 per cent of social disparities and 72 per cent of academic differences among classes in the same establishment arise from the random element in the constitution of classes. Because chance does not “naturally” create diversity, greater attention by school heads to the academic and social balance of their classes would help to reduce significantly the levels of segregation. It should be noted, however, that beyond this “passive” segregation, from 15 to 20 per cent of schools engage in “active” segregation by grouping students according to their stream and subject choices. This is particularly evident in the municipalities and counties that are richer than average, and where public schools compete with private schools.