The COVID-19 pandemic led to numerous governments deciding to close schools for several weeks in spring 2020. Empirical evidence on the impact of COVID-19-related school closures on academic achievement is only just emerging in the literature. The present work aimed to provide a first systematic overview of evidence-based studies on general and differential effects of COVID-19-related school closures in spring 2020 on student achievement in K–12. The findings indicate a considerably negative effect of school closures on student achievement specifically in younger students and students from families with low socioeconomic status. At the same time, certain measures can be identified that might mitigate these negative effects. The findings are discussed in the context of their possible consequences for future national educational policies when facing future school closures.