French secondary schools may differ widely as far as their intake is concerned, due to both school mapping and, to some degree, school choice at that level. For instance, the percentage of pupils from high SES may vary from 0% to 90,4%. But, until now French sociology has been more interested in investigating individual factors underlying social inequalities rather than contextual ones. While, after Bourdieu's research, we know relatively well the mechanisms explaining social inequalities in academic achievement and orientation processes, we remain more ignorant concerning the role played by school factors. In France, the centralisation of the system leads to consider that schools tend to be homogenous across the country, and very few empirical studies have tried to verify this common view.We know that in France, school has a specific impact on pupils performances, at the level of the collège (junior secondary schools) (Duru-Bellat & Mingat, 1988), even at the level of the lycée (upper secondary schools) (Le Bastard-Landrier, 2002). Thrupp (1999) studies suggest that " many school processes which have been identified as contributing to student achievement may be less independent of school mix than researcher have typically allowed ". As well, Opdenakker & Van Damme (2001) research confirms that there are important relationships between pupils' SES and school process variables in secondary education. Until today, no such studies have been carried out in France. This is why this exploratory research has been undertaken in 2002-2003.This communication will present this on-going research aiming at assessing the impact of school mix (both social and academic mix) on achievement of pupils from French lycées. This study is developing both a quantitative and qualitative approach : it is based on a rather small sample (27 schools, about 2000 pupils), but it tries to include a variety of qualitative data to assess the processes through which social mix might have an influence. We will analyse the relations between school mix and the management of a school and its organisation, the quantity and quality of instruction, and pupils' behaviour. We have carried out interviews with headmasters (role of headmasters, stability of the teaching staff, characteristics of the pupils, organisation of the school...). Questionnaires were submitted to pupils (peer's relationships, attitudes with respect to authority and norms in schools...) and we made qualitative observations about the quality and cleanliness of the buildings, playgrounds, classrooms, pupils' common rooms within the lycées surveyed. We will also present results concerning the impact of school mix on pupils' achievement in french and mathematics, as well as the first qualitative results concerning pupils' behaviour and norms between socially contrasted schools.