New York City will reopen its school system to in-person learning, and increase the number of days a week many children attend class, even as the coronavirus pandemic intensifies in the city, Mayor Bill de Blasio said Sunday.

The announcement marks a major policy reversal for the nation’s largest school system, less than two weeks after de Blasio announced that schools were shutting down because of a rising number of COVID-19 cases in the city.

Some elementary schools and pre-kindergarten programs will resume classes Dec. 7, a week from Monday, the mayor said. Others will take longer to reopen their doors.

School programs serving special-needs students at all grade levels will open to in-person learning starting Dec. 10, de Blasio said.

The plan for reopening middle and high schools is still being developed, de Blasio said.

“We feel confident that we can keep schools safe,” he said.

Gov. Andrew Cuomo, a fellow Democrat, said he supports de Blasio’s school reopening plan.

“I think that’s the right direction,” the governor said in a conference call with reporters. “We do have new facts and information on schools.”

New York City’s public schools opened to in-person learning starting in September for students whose parents had chosen bricks-and-mortar schooling. School buildings closed again Nov. 19 amid rising COVID-19 infections in the city.

Masks and social distancing were mandatory during the weeks that schools were open, and class sizes were a fraction of the pre-pandemic average of up to 25 to 30 students.

About 190,000 students will be eligible to return to classrooms in the first round of reopening, just a fraction of the more than 1 million total pupils in the system. The great majority of parents have opted to have their kids learn remotely by computer.

De Blasio said that many of those returning in person will be able to attend five days of class a week, up from one to three days previously.

Students attending in person will be required to undergo frequent testing for the virus. Previously, the city had set a target of testing 20% of teachers and students in each school building once a month. Now, the testing will be weekly.

The mayor said the city was doing away with its previous trigger for closing schools, which was when 3% or more of the virus tests conducted in the city over a seven-day period came back positive.

“The idea of the hard number made a lot of sense back in the summer when we had not yet experienced all this,” de Blasio said.

Since then, de Blasio suggested, relatively low numbers of positive coronavirus tests at schools show that it’s possible to keep schools open even with a citywide test positivity rate over 3%.

Schools that are in COVID-19 orange zones designated by Cuomo because of rising infection rates will reopen according to rules set by the governor, de Blasio said.

De Blasio said at a news briefing that he had discussed his reopening plan with Cuomo. “We all agree that we have a different reality than what we had in the summer and that this is now the way forward and the best way to protect everyone,” he said.

Michael Mulgrew, the president of the United Federation of Teachers, said the union supports the reopening plan as long as stringent testing is in place.

“This strategy — properly implemented — will allow us to offer safe in-person instruction to the maximum number of students until we beat the pandemic,” Mulgrew said in a statement.

Mark Cannizzaro, the president of the Council of School Supervisors and Administrators, which represents principals, said school leaders “deeply understand the need for children to learn in person as regularly as possible, so long as all safety protocols are based on the guidance of medical professionals.”

A parents group called #KeepNYCSchoolsOpen praised the mayor’s reopening plan but said it doesn’t go far enough. “Our schools never should have closed, and we are thrilled that 190,000 kids will be back in school next week,” the group said in a statement. “But the mayor has left behind 145,000 kids in grades 6-12, without any explanation or timeline for reopening. We will not rest until they are back in school, where they need to be.”

New York City exceeded the 3% threshold early in November, and things have slightly worsened since then. More than 9,300 New York City residents have tested positive for the virus over the past seven days.

Rates of positive coronavirus tests at school sites have remained low, however. A spokesperson for de Blasio said that more than 160,000 students and school staff members have been tested and that only about 0.25% of the results were positive.