In this article, we address three broad conjectures about what really matters with respect to school effectiveness. Our review of previous evidence prompted us to look at three sets of factors connected with classroom teachers, school policies and processes, and matters of governance. All three have featured prominently in the public arena. In particular, we look for the relative contributions of teacher-, school-, and governance indicators for educational effectiveness (measured by Math achievement). About 100 Dutch primary schools form the database together with findings of international school effectiveness research (studies, reviews, and meta-analyses). School-level variables are the most substantial in explaining educational effectiveness. The sector effect (public/private) explains 16% of the between school variance, other school-level variables explain 51%, and the teacher- or classroom-level variables explain 32%. Some of the underlying variables are identified and we address three broad conjectures about what really matters with respect to school effectiveness.