In the Seattle area, there have been several short-term school closures for cleaning. But the Northshore School District, after several individual school closures, announced it was closing the entire district for up to two weeks. Its more than 23,000 students will convert to online learning in the meantime.

And at Saint Raphael Academy, a private Catholic school in Pawtucket, R.I., administrators announced that the school would be closed this week after chaperones and a student who had traveled to Italy on a school trip tested positive for the virus.

The situation is “like a huge snowstorm or a hurricane, in that it requires calm, and it requires people to dig deep and look out for each other, but it’s different,” said Holly Taylor Coolman, 53, whose two teenagers at the school will now be switching to “virtual learning” for the next two weeks. “I’m trying myself to really look for the positives,” she said.

It is not clear how effective school closings are during virus outbreaks, said Dr. Jeff Duchin, health officer for the Seattle and King County Public Health agency. Diseases spread faster among people in close quarters, a factor that often drives decisions to close schools. But schools in the Seattle area closed during the H1N1 pandemic a decade ago, and “we saw tremendous community disruption,” Dr. Duchin said.

Students continued to gather elsewhere, he said, and parents were burdened by having to stay home from work. Among them were health care workers who were unable to provide care because they had to attend to their children.

Many lower-income students also rely on schools for meals. Officials in New York City have said that they would close the public schools there only as a last resort, in part because about 114,000 students in the school system are homeless and may have nowhere else to get hot meals, medical care or even a place to wash their dirty laundry.

State and federal officials have yet to require school closures, so for now the decisions are in the hands of individual districts and schools.