Since the COVID-19 pandemic emerged in March 2020, districts across the nation have faced the difficult task of reopening school sites safely for in-person instruction and keeping them open as community infection rates have risen and fallen. It is useful to learn from the efforts of districts that have been able to reopen schools--and keep them open--using multilayered mitigation strategies that reduce the risk of in-school transmission. This brief describes reopening efforts in our nation's largest district, New York City, which has, with some brief interruptions, maintained in-person schooling for hundreds of thousands of elementary-age students since late September 2020. Strict implementation of mitigation strategies--including masking, adequate ventilation, physical distancing, symptom screening, and testing and contact tracing--have made this work possible. This brief is part of an effort at the Learning Policy Institute to disseminate key public health research and reopening strategies to educators and policymakers.