The international and comparative education literature is not in agreement over the role of schools in student learning. The authors reexamine this debate across 25 diverse countries participating in the fourth-grade application of the 2003 Trends in International Mathematics and Science Study. The authors find the following: (a) In most cases, family background is more important than schools in understanding variations in student performance; (b) schools are nonetheless a significant source of variation in student performance, especially in poor and unequal countries; (c) in some cases, schools may bridge the achievement gap between high and low socioeconomic status children. However, schools' ability to do so is not systematically related to a country's economic or inequality status. (Contains 8 tables and 15 notes.)