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 international variation in school closure policies to examine the effects of school closures on student achievement. Specifically, we use representative trend data from more than 300,000 students in 29 countries to examine whether 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 school closures on student reading achievement. The estimated effect implies that a year of school closures corresponds roughly to the loss of a little more than half a school year of learning. This effect is even more pronounced for socioeconomically disadvantaged students and those without home computer access.