Even if the literature on the effects of pupil composition has been extensive, no clear consensus has been reached concerning the significance and magnitude of this effect. The first objective of this article is to estimate the magnitude of the school composition effect in primary schools (6th grade) in French-speaking Belgium. Different indicators of school composition are used: academic, socio-cultural, "language" and sex composition. Except for sex composition, the results show that the school composition effect explains significant amount of between schools variance even after controlling for pupils' initial performance, socio-cultural background, and non-cognitive dispositions. The second objective is to examine covariance between school composition and several organisational variables and their joint effect on school performance. The second set of analyses is intended to question the conceptual nature of the school composition effect, establishing whether it is direct or indirect.