School composition effects have been consistently verified, but few studies ever attempted to study how school composition affects school achievement. Based on prior research findings, we employed multilevel mediation modeling to examine whether school processes mediate the effect of school composition upon school outcomes based on the data of 28 OECD countries in PISA 2003. These school process factors include school resources, opportunity to learn, school climate and school management. We find that school composition effects are partially mediated by school climate factors.