The gross differences in scholastic achievement among public, private government-dependent, and private independent schools in 22 countries are analyzed with Programme for International Student Assessment 2000 data. In a multilevel approach, the authors estimate these sector effects, controlling for sociological characteristics of students and parents, school composition, teaching and learning conditions of schools, and students' and principals' perception of the climate of their schools. The main explanation of their gross differences in scholastic achievement is the better social composition of private schools, both government dependent and independent. But pupils at private government-dependent schools have a higher net educational achievement than do comparable pupils at public schools with the same social composition. The explanation of these remaining net differences in scholastic achievement seems to be their better school climate. These net differences in scholastic achievement between public and private school sectors are equal across nations, despite historical differences of educational systems. (Contains 5 tables and 10 notes.)