One of the first school districts in the country to reopen its doors during the coronavirus pandemic did not even make it a day before being forced to grapple with the issue facing every system actively trying to get students into classrooms: What happens when someone comes to school infected?

Just hours into the first day of classes on Thursday, a call from the county health department notified Greenfield Central Junior High School in Indiana that a student who had walked the halls and sat in various classrooms had tested positive for the coronavirus.

Administrators began an emergency protocol, isolating the student and ordering everyone who had come into close contact with the person, including other students, to quarantine for 14 days. It is unclear whether the student infected anyone else.... on Day 1.”

To avoid the same scenario, hundreds of districts across the country that were once planning to reopen their classrooms, many on a part-time basis, have reversed course in recent weeks as infections have spiked in many states.

Those that do still reopen are having to prepare for the near-certain likelihood of quarantines and abrupt shutdowns when students and staff members test positive.

Of the nation’s 25 largest school districts, all but six have announced they will start remotely, although some in places like Florida and Texas are hoping to open classrooms after a few weeks if infection rates go down, over strong objections from teachers’ unions.