The Belgian educational field includes separate educational systems reflecting the division of the country into linguistic communities. Even if the French-speaking and the Dutch-speaking communities keep sharing important similarities in terms of funding rules and structures, they present a huge gap between their respective pupils' achievement. The Belgian educational systems are then characterized by high levels of segregation; this paper aims to model the potential differential effect of school composition on pupils' achievement. Multilevel models are consequently tested on the Programme for International Student Assessment 2009 data (7184 respondents in 236 schools). Our results suggest that the position of pupils in the educational system still has a deep influence in a system that functions as a quasi-market and where grade repetition and tracking are widely used. Our analysis also confirms that academic and socio-economic compositions have an extra negative effect on pupils' achievement.