The COVID-19 pandemic implied measures, such as school closures at the outbreak of the pandemic, that negatively affected children's human capital. However, in some places, the situation later brought a reduction in class sizes, in order to avoid the spread of SARS-Cov-2 at schools. We exploit this unexpected event to evaluate the effect on school performance of a sizable class size reduction implemented in Spain, when schools were reopened. We find a positive and significant effect of the class size reduction of 0.11 standard deviations on overall students performance. Given the situation and nature of our data, we interpret our estimates as a lower bound for the true effect of the reduction in class size. Our conclusion is that the reduction in class size served, on average, to at least compensate the other negative effects for learning. Our findings also point out at the importance of evaluating the quality of the new and unexperienced additional teachers that need to be hired when implementing a general reduction of class size.We are grateful to the Consejeria de Educación y Empleo of the regional Government of Extremadura (Spain) and the employees there that authorized and helped accessing the data. All remaining errors and responsibility for this work are our own. The fi rst author gratefully acknowledge research funding from the Agencia Estatal de Investigación del Gobierno de España, grant RTI2018-095231-B-I00, and Comunidad de Madrid (Spain), grant EPUC3M11 (V PRICIT). There is no other fi nantial sources nor potential con ict of interest to declare
