The spread of the novel coronavirus has prompted schools across the nation to close for days or weeks at a time, shutting tens of thousands of students out of their classrooms while officials in all parts of the country—even those untouched by the virus—brace for the possibility of closing their doors.

More than 380 schools have closed their doors because of the outbreak, moves that have affected nearly 260,000 students, according to a count by the education publication EdWeek. Many of those closures were for just a day or two so workers could deep-clean schools where officials worried about exposure to the virus.

Many others, heeding the advice of the Education Department and the Centers for Disease Control and Prevention, were preparing for potential for long-term closures. Some systems closed schools to train teachers and prepare lessons in case students have to learn remotely.

Among the closures were all Avon Community School Corp. schools, which will be closed through March 20 after a second student began displaying symptoms of COVID-19, the school system announced Monday afternoon.

While many closures will be short-lived, some districts, especially those close to larger outbreaks, announced longer-term closures. Northshore School District educates more than 23,000 students near Kirkland, Washington, where a nursing home has been linked to 14 coronavirus deaths.

Michelle Reid, superintendent of the school system, said she made the call when she learned that 500 members of her staff—including bus drivers, veteran teachers and pregnant administrators—were at risk of developing serious complications if they contracted the virus.

“I believe they’re superheroes, but they’re not immune to this virus,” Reid said. “I concluded there was no way I could safely operate the district.”

Globally, 14 countries—including China, Italy, Japan and South Korea—closed all of their schools, leaving more than 290 million children out of the classroom, according to the United Nations. Thirteen other countries, including the United States, closed some schools.

U.S. school officials in many places are grappling with how to proceed, weighing the potential benefits of closures in slowing the spread of the disease against the severe disruption it could cause, particularly for students who rely on schools for meals, or who have parents who would struggle to find child care. There are also questions of how effective school closures are in slowing down the spread of disease, especially if people congregate elsewhere and pass the illness on to one another outside of the schools.

In Hastings-on-Hudson, New York, just north of New York City, the school district canceled events and closed for two days last week after the middle school principal learned that a parent at her school worked in a nearby school where a student had tested positive for covid-19, said Valerie Henning-Piedmonte, the Hastings-on-Hudson superintendent. She worried that the middle school mother might have been infected and then perhaps infected her own children, who might have brought the virus into the district.

She also convened her district’s safety committee, and heard from school nurses, who said they worried that children and staffers with underlying health issues would be at particular risk with possible exposure. That sort of anxiety helped persuade her to close for two days.

“If this is known and confirmed and we don’t respond in some way, it’s going to sort of snowball,” she said. “The best thing to do was to hit the pause button, disinfect and sanitize the building and get more information.”

The Elk Grove Unified School District, near Sacramento, California, shuttered all of its schools for a week because two members of a household with children in its schools had tested positive for covid-19. One of the students, in elementary school, has since tested positive for the virus.

But school officials are operating with many unknowns as doctors continue to learn how the disease manifests in children and how easily they can spread it. There have been relatively few cases of children with covid-19, and those who did get it were far less likely to develop serious complications, according to the World Health Organization. China reported no deaths of young children linked to the coronavirus.

On Monday, Fulton County Schools, which educates more than 95,000 students in the Atlanta area, announced that it would close for cleaning “affected schools” on Tuesday after a school employee was hospitalized with covid-19.

In New York, home to the nation’s largest school district, Mayor Bill de Blasio, a Democrat, said he was reluctant to close schools and would only do so on a targeted, limited basis – what he called a “pinpoint response.”

“Of course safety first, but they depend on the schools, they are a safe place for their kids and by the way, they want their kids to keep getting educated. So to me it’s a high bar for a closure,” de Blasio said Monday on CNN.

Howard Markel, director of the Center for the History of Medicine at the University of Michigan, said research on past pandemics shows that closing schools can help stop the spread of infectious disease but only if administrators act early.

“You have to do it before it spreads,” he said. “Young kids sneeze and cough and slobber on each other.” He said that even in an uncrowded classroom, children are close enough to spread disease. “There’s lots of reasons to consider school closure today.”

Others caution against that action, saying there is no evidence it will help stem the spread of the virus.

“I have one foot deeper into ‘don’t close schools’ right now than ‘close them,’ ” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “And I can’t even say closing down schools to clean them will make a difference. The data isn’t there.”

And several pointed out the downsides to closing schools. If students get together outside of school, it may have little effect on slowing transmission. And some students rely on school not just for education but for meals and health care.

Osterholm said an estimated one-third or more of women in the United States who are nurses are also primary-care providers for children younger than 18.

“If we take these nurses out of work, we are not just talking about closing schools but also about the impact we are having on health care, which can be dramatic,” Osterholm said. “It is too knee-jerk a reaction to say just close schools out of an abundance of caution when there are potentially serious downsides to this.”

Laurie Combe, president of the National Association of School Nurses who served in Houston-area schools for 25 years, said officials should also consider the fact that sending students home could make it harder for them to access health care, with some uninsured children relying on school nurses and school-based health clinics.

The outbreak near Seattle has led to several large employers sending workers home to work remotely. In some households, that means every family member is now cooped up at home – parents telecommuting and children engaged in online lessons because their schools are closed.

Alisa Shtromberg, her husband and two children are all working, studying and playing from home this week. Shtromberg is a web content manager for a community college that has closed, while her children attend Northshore schools.

Covid-19 has also shut down the family’s social life. When her 13-year-old son asked to hang out with friends, she told him no. She is still considering whether to send her younger son to a birthday party this month.

“We are a very active family,” Shtromberg said, adding that her sons’ schedules are normally packed with extracurriculars, most of which have been canceled. “So for us to basically be more or less at home the bulk of the time has been really tough.”

Shtromberg said she was glad the school district shut down; she was more concerned for the well-being of older family members and what would happen if she or her sons inadvertently passed on the illness.

For Northshore schools, Reid said she considered how to minimize the effects of the closure. The school district is affluent, populated with families employed by Amazon, Boeing and Microsoft, and has moved classes to the web. Still, about 1 in 6 children qualifies for a free or reduced-price meal because of family income.

So the district set up regional food distribution centers, where families could go for a free meal. It furnished laptops and WiFi hotspots to families who don’t have computers or Internet access. And it has set up a team to address “social isolation” for students who may get lonely during their time out of school.

But she acknowledged that her district would not weather the crisis as easily if it were not so well-resourced.

“We do have resources that other communities do not have,” Reid said. “It just illuminates the inequities that schools have just based on their Zip codes.”