In the scientific literature, two kinds of determinants have successively been proposed and opposed to explain differentiated school effectiveness: school composition and school processes. The first aim of this article is to shed light on that debate, by reviewing qualitative studies showing how managerial and pedagogical processes are connected to school composition. After that, using PISA 2000 mathematics scores, a rotation technique of multi-level models is conducted to estimate the net effect of school composition, the net effect of school processes and the joint effect of those two dimensions. Five European education systems (Flemish and French Belgium, France, Austria, Portugal) characterized by a strong between-schools variance (more than twenty per cent of the total variance of the mathematics scores) are taken into account. The results suggest that the joint effect of school composition and school processes explains a significant part of the between-schools variance. The implications of these results for empirical and theoretical status of schools and for accountability policies are then discussed.