Until now, French sociology has been more interested by investigating individual factors underlying social inequalities than by contextual ones. After Bourdieu's research, we know relatively well the mechanisms explaining social inequalities in academic achievement and orientation processes. But, we remain more ignorant concerning the role played by school factors. In France, the centralization of the system leads to consider that schools seem to be more or less homogeneous, and very few empirical studies have tried to verify this common view. However, due to school mapping, French primary schools may differ widely as far as their intake is concerned. For instance, the percentage of pupils from high SES can vary from 0% to more than 80%. Thus, the effects of school, class, or teacher, on pupils' achievement were identified relatively recently and we don't really know what are the processes which could explain these contextual effects.This communication will present an on- going research aiming at assessing the impact of school mix (both social and academic mix) on achievement of pupils of French primary schools. Indeed, according to researchers of other countries (Thrupp, 1999 ; Opdenakker & Van Damme, 2001) school mix could be a significant explanation of contextual effects. This research is developing both a quantitative and qualitative approach around three major questions : are there tangible school mix effects at the primary level, are these effects different with respect to pupils' SES and what are the processes through which social mix might have some influence. The quantitative analysis is based on a sample of 91 classes of CE1 (grade 2) and 87 classes of CM1 (grade 4) and includes about 2000 pupils at each level. Moreover, we have carried out interviews with headmasters, submitted questionnaires to pupils and teachers and made some qualitative observations within 10 socially contrasted schools. The aim is to analyse the relations between school mix and the management of the school and its organization, the quantity and quality of instruction, and pupils' behaviour.In the first place, we will present the results of the quantitative analysis by focusing on the measure of the school mix impact on pupils achievement in french and mathematics. In the second place, we will present the results of the qualitative approach. We will focus on results concerning the differences of pupils' behaviour within socially contrasted schools. We will also deal with the issue of teachers' practices especially in terms of teaching content, discipline management, assessment, expectations and relations between teachers.