In New York City, the nation’s largest school district has lost some 50,000 students over the past two years. In Michigan, enrollment remains more than 50,000 below prepandemic levels from big cities to the rural Upper Peninsula. In the suburbs of Orange County, Calif., where families have moved for generations to be part of the public school system, enrollment slid for the second consecutive year; statewide, more than a quarter-million public school students have dropped from California’s rolls since 2019. And since school funding is tied to enrollment, cities that have lost many students — including Denver, Albuquerque and Oakland...