This article analyzes the factors that explain the gap in educational outcomes between the top and bottom quartile of students in different countries, according to their socioeconomic status. To do so, it uses PISA microdata for 10 middle-income and 2 high-income countries, and applies the Oaxaca-Blinder decomposition method. Its results show that students' individual variables only explain differences in high-income countries; meanwhile, school and teacher quality, and better practices, matter even in different institutional settings. From a policy perspective, this evidence supports actions to improve school and teacher quality in order to reduce cross-country differences and differences between students at the top and bottom of socioeconomic distribution.