Even in places with scant evidence of the disease, governors were ordering shutdowns. That left school leaders scrambling to arrange online learning, meal distribution and accommodations for children with disabilities, with many systems writing plans on the fly.

On Friday, school leaders in the District of Columbia, Los Angeles and San Diego and governors in Pennsylvania, Illinois, Virginia, Wisconsin, Louisiana, Alabama, Utah and West Virginia joined five other states in closing their entire school systems.

“There is a downside to this, but it is the right thing in my mind to do,” West Virginia Gov. Jim Justice (R) said. At the time of his announcement, his state did not have a confirmed case of coronavirus, but he said: “We’ve got a monster that is looming.”

In New York City, the nation’s largest district with more than 1 million students, pressure built to close the schools, with calls for closure from the United Federation of Teachers and the speaker of the New York City Council, Corey Johnson, who said on Twitter: “It is not time to panic but it is time to act.” An online petition urging closure had more than 235,000 signatures.