The U.S. has entered a second round of back-to-school, just as the coronavirus surges around the nation. In smaller school districts, careful in-person reopenings in August and September didn’t lead to an explosion of COVID-19 cases. And now, the country's largest school systems, which had largely eschewed in-person instruction, are venturing partially back into the classroom. The majority of the 15 largest districts in the nation now have at least some students in school buildings. Only two of those districts had any form of in-person learning as of early September.