The Wallonia-Brussels Federation (FWB, Belgium) is deeply marked by academic and socioeconomic segregation which stems from the particular way in which its education system is organised. School segregation has been widely highlighted. Class segregation (within schools) has received little attention in the literature, but is a significant phenomenon. Firstly, the input of different measures and data used to measure school segregation are presented. Secondly, an index based on multilevel analysis is proposed to simultaneously measure segregation between schools and within schools. It was applied to a sample of 10,309 students in the second common year (of mainstream secondary education) in 591 classes and 97 schools in the FWB. The analysis shows significant segregation, especially between classes within schools.