The spread of coronavirus has compelled hundreds of K-12 schools in the U.S. to close, affecting more than 850,000 students, according to an analysis by Education Week. And those numbers are certain to increase in the coming days, as concerned parents call for more school closures.

The growing health crisis presents school leaders with a painful choice. Closing schools — as has been done, so far, in China, Japan, Italy and elsewhere — is a proven measure that has been shown to slow the spread of disease and, in turn, save lives. But it also causes huge economic and social disruption, especially for children, millions of whom depend on the free and reduced-cost meals they get at school.

Public officials understandably don't want to close schools unless they absolutely have to, and many closures so far have been triggered by a known case of infection or exposure among staff or students. Yet research suggests the best time to close schools is before that happens.

The fact remains that canceling school is a difficult decision because no one knows better than educators just how much some children depend on the support they get there.

"For a large number of our students, the safest place for them to be is actually in school," says Sonja Santelises, CEO of Baltimore City Public Schools. That's why, she says, she resists canceling school because of weather, too. "It gets me some angry emails during snowstorms and inclement weather. ... I will just tell you, our mantra continues to be: Closing schools is a last resort."

One big concern for school leaders is that many kids would be going home to empty households.

"Yeah, I think that's the hardest contemplation for our districts," says Chris Reykdal, the superintendent of public instruction for Washington state, which has been hit hard by coronavirus. He's concerned about sending "a million Washington kids home knowing that for hundreds of thousands of them, they simply will not have any parents at home."

Reykdal says that when his schools take a snow day, parents can sometimes make do for a day or two, either taking the time off or working from home. But coronavirus isn't snow, and there's no way to know how long schools would have to close. Many employers may continue to require employees to come to work. (Think about the children of health care workers.) And without government intervention, which is on the table, many low-wage workers won't be able to afford to stay home.