Coronavirus Live Updates: Newsom Announces Rules Forcing Most California Schools to Start Remotely

Education leaders in Houston, Atlanta, Nashville, Arlington, Va., and Broward County, Fla., also said this week that they planned to open the academic year online, despite pressure from President Trump and some Republican governors who want students in their classrooms five days a week.

In Texas, where state officials had previously put limits on online schooling, new guidelines were issued Friday that would allow as many as eight weeks of online-only instruction when schools come back in session next month.

And leaders of Chicago’s public school system, the nation’s third-largest district after New York and Los Angeles, said on Friday that they’re planning for a mix of in-person and online classes. But they stressed that the framework was tentative, with a final plan expected in August. New York City schools are also planning an in-person and online mix.

The California rules announced on Friday would force schools in counties that the state has put on a “watchlist” — because the virus is spreading rapidly there — to teach online until they meet certain public health thresholds. Currently, 32 of the state’s 58 counties, including many of the most populated, are on that list.

The rules would also require teachers and staff to maintain six feet of physical distance in schools that are allowed to reopen, and mandate masks for students in third grade and up. Younger children would be encouraged but not required to wear face coverings.

The guidelines also recommend that school employees be tested regularly for the coronavirus, something teachers across the country have been pushing for, although the Centers for Disease Control and Prevention has said doing so is not necessary, and scaling up testing has been a challenge.

The C.D.C. said Friday that it would not release its guidance for reopening schools this week as expected, saying that they would be released by the end of the month. Mr. Trump clashed with the C.D.C. earlier this month, calling its proposed guidelines for schools “very tough and expensive” and demanding that they be revised.