The COVID-19 pandemic disrupted education worldwide as educational systems made the decision to close schools to contain the spread of the virus. The duration of school closures varied greatly internationally. In this study, we use representative trend data from more than 300,000 students in 29 countries to examine whether the cross-country variation in the length of school closures is related to changes in student achievement before and after the outbreak of COVID-19. We observe a significant and substantial negative effect of longer school closures on student reading achievement. This effect is even more pronounced for socioeconomically disadvantaged students and those without home computer access.