Friday, Mayor de Blasio and other city officials laid out in detail what will happen when a student, teacher or staff member tests positive for COVID-19.

Critics say by then, it may be too late to contain the spread within a school.

"There has to be intermittent, random testing of each school community, which we don't have,” said Michael Mulgrew, president of the United Federation of Teachers. “The reason we asked for that is because all the medical professionals who we're dealing with have told us that that's imperative."

The UFT would also like to see more vigorous contact tracing throughout a school when a student tests positive.

While the city will prioritize free testing for school staff, with a 24-hour turnaround, regular, monthly testing will only be encouraged, not required.

"We know a lot of people want to get that testing,” de Blasio said Friday. “And so that's what we're going to keep doing: constantly making it available for free. And we think a lot of people take us up on that."

City Councilman Mark Treyger of Brooklyn, who chairs the Council's education committee, questions the price tag of the city's protective measures.

"I am told by school cleaners as of this morning that they don't have the money to do deep cleaning of buildings every night,” Treyger said. "I’m also told that there is a back order on Lysol wipes that could be potentially five months long."

Others critics say only regular testing will give schools a chance of catching infections before they multiply, and averting a situation that requires a full school closure.

"How can you test somebody at the beginning of the school year, and never require testing throughout the week?” City Comptroller Scott Stringer said. “We will not know if there's an outbreak, or if the schools are being seeded with the disease."

Stringer and others have pushed for more creative use of outdoor space, like appropriating city streets in the area surrounding schools. De Blasio said that’s not a reliable option.

“I would remind you that outdoors might seem really appealing in September or June, but it's not going to be as workable, say, in November, December, January, February, March,” he said. “It's unpredictable to say the least."