On a more positive note, Mr. Kosuth said that evidence from China suggested that children were more resilient to the coronavirus than adults were.

In Miami-Dade County, Fla., Alberto M. Carvalho, superintendent of one of the nation’s largest school districts, said his system’s preparation for hurricanes put it at an advantage in preparing for the coronavirus. The district has provided laptops, tablets and smartphones for some students to take home, as well as internet connectivity for some low-income students. Teachers would be asked to assign work remotely and could even teach some high school courses live online.

“I was a bit surprised that it took this long to offer national guidance specifically to school districts,” Mr. Carvalho said of the C.D.C. statement this week.

Many districts have already sent home letters about the coronavirus, asking parents to keep sick children away from school and to remember basic prevention measures such as hand washing, cough covering and vaccination against the flu. They have highlighted C.D.C. advice issued early this month, calling for all travelers returning from China to “self-quarantine” for 14 days.

School officials have often tried to ratchet down panic among parents, reminding families that face masks are not broadly recommended and that the overall risk of infection is low.

But few districts have publicly addressed what would happen to classes in the case of widespread infection and school closings like those that have taken place in China, Italy and Bahrain.

[Read: Japan shocks parents by moving to close all schools in response to coronavirus.]

The vast majority of districts have access to broadband internet, but they do not necessarily have expertise in how to effectively organize and teach classes online when schools are shuttered. Further complicating matters, not all families have home computers and high-speed internet. While 90 percent of households with children under 18 had broadband access in 2016, according to federal data, gaps remained along the lines of income, race and education level.