A central objective of schooling processes at the international level is reading skills development. Unfortunately, many students in the European Union underperform at this, and these low performances can be more pronounced in countries with lower Human Development Index (HDI) values. This study analysed the contextual effect of school socioeconomic status (SES) on reading performance using PISA data from 27 countries. We found that school SES had a positive contextual effect on student reading performance, especially countries with lower HDI values, via reading self-competence. However, in middle and higher HDI countries, the contextual effect was smaller, and we did not observe an indirect effect via reading self-competence. We conclude that school SES, rather than student SES, matters the most. Therefore, the modification of school composition and resources might act as a buffer for low student SES, particularly in lower HDI countries.