[This briefing has ended. For the latest updates on the coronavirus outbreak in the New York area, read Saturday’s live coverage.]

Mr. de Blasio, speaking at an evening news conference, said the question of whether to keep the schools open was more complex than some people were making it seem. Mass closings, he said, could shut schools down not just for the rest of the current academic year but potentially through the rest of 2020.

“We shut down the school system, we might not see it for the rest of the school year, we might not see the beginning of the new school year,” he said. “And that weighs heavily on me.”

Gov. Andrew M. Cuomo has also resisted calls to close public schools across the state.

Even public health experts with dissenting views on whether entire school systems should be closed say that to be effective in protecting public health, such moves should be long-term, ideally for as long a threat like the novel coronavirus persists.

The federal Centers for Disease Control said on Friday that “short to medium closures” did not have an impact on mitigating the spread of the virus, but that closings of eight to 20 weeks might have “some impact.” The C.D.C. also said that other countries that closed schools did not necessarily have more success in curbing the virus’s spread than those that did not.