New York City will reopen its school system to in-person learning, and increase the number of days a week many children attend class, even as the coronavirus pandemic intensifies in the city, Mayor Bill de Blasio said Sunday. The announcement marks a major policy reversal for the nation’s largest school system, less than two weeks after de Blasio announced that schools were shutting down because of a rising number of COVID-19 cases in the city. Some elementary schools and pre-kindergarten programs will resume classes Dec. 7, a week from Monday, the mayor said.