Social and ethnic segregation is particularly high in the educational systems of countries like Belgium, Germany, France, Austria and the Netherlands. This EQUOP project seeks to disentangle what are the crucial factors by which this high level of segregation impacts on unequal opportunities for children. Our empirical focus will be on Belgium and its Flemish and Francophone educational systems. Going beyond the classic composition effect model (looking at peer group effects, i.e. positive or negative influences of pupils on each other), this project wants to also examine the potential impact of differential strength of teacher teams on pupils' performance. The project wishes to test the hypothesis that the link between school composition and educational performance is a (partly) spurious effect, caused by a mediating effect of characteristics of teacher teams. We hypothesize that better skilled and more positively oriented teachers are overrepresented in schools with an 'easier' school population, while so-called 'difficult' schools (populated by working-class immigrant children) have difficulty in attracting and - especially - keeping competent and motivated staff and building a strong collective culture at school level. At the same time we suspect that well performing schools with high numbers of at-risk pupils have a stronger and more stable teacher team. In order to examine this hypothesis a mixed methods approach will be used, combining quantitative statistical analysis (on new and existing data, for instance multi-level analysis of the PISA-data set and other eligible datasets), qualitative case studies and focus groups. Secondary analysis of existing data-sets (PISA, TIMMS and PIRLS) will be undertaken and new data will be collected (taking the Flemish and Francophone educational systems in Belgium as case-studies).