NEW YORK - In-person learning at public schools will be suspended starting Thursday after the city's coronavirus infection rate reached 3%, Mayor Bill de Blasio announced first on Twitter and then in his daily press briefing, which occurred more than five hours later than its original start time.

It's a big blow for the nation's largest school system, which pushed hard to reopen for in-person learning in September following a spring of remote instruction. 

"New York City has reached the 3% testing positivity 7-day average threshold. Unfortunately, this means public school buildings will be closed as of tomorrow, Thursday Nov. 19, out an abundance of caution. We must fight back the second wave of COVID-19," de Blasio tweeted. 

While parents and school staff waited for the mayor to begin his press conference, Gov. Andrew Cuomo held one at 1:30 p.m., presenting the state's numbers, which have always been different than the city's numbers. The state's seven-day rate for the city stood at 2.5%, below the 3% threshold the city said it had reached. The city measures results by the date a coronavirus test was taken as opposed to the date the test results come in, according to the mayor. 

The mayor said his late start time was due to the city working to confirm the 3% threshold had indeed been met and to communicate restrictions with the governor's office.

"No one is happy about this decision," he said. "We all, in fact, are feeling very sad about this decision because so much good work has been put into keeping the schools - opening them up to begin with, lets start there - opening the schools when almost no other major school system in America opened, making them so safe. But we set a very clear standard and we need to stick to that standard."

The decision throws into chaos the schedules of the nearly 300,000 students who have enrolled in blended learning, which offers both in-person and remote instruction. 

In a letter to principals obtained by NY1 prior to the announcement, Schools Chancellor Richard Carranza wrote:

"As of this morning, November 18, the City has now reached this threshold of test positivity citywide and, as a result, the DOE will temporarily close down all public school buildings for in-person learning, Thursday, November 19. This action, along with other city-wide measures, is a key component to address the concerning rise in COVID-19 transmission rates. This closure of buildings is temporary; we will work diligently alongside other City agencies and every New Yorker to bring this transmission rate back down and get back to in-person learning as quickly and as safely as possible."

Over the past week as coronavirus cases continued to rise in the city, the mayor repeatedly said he would stick to that 3% threshold to determine whether to shut down schools. The threshold was set prior to the start of the school year as a way to reassure parents, students and the teachers union that the city was taking their safety seriously. 

"The city established the three percent infection rate threshold to make sure that schools did not become centers to spread the coronavirus. Since the three percent rate has been reached, education will continue but all students will be learning remotely," teachers union president Michael Mulgrew said in a statement. "Now it’s the job of all New Yorkers to maintain social distance, wear masks and take all other steps to substantially lower the infection rate so school buildings can re-open for in-person instruction."

Mayor de Blasio assured parents, students and teachers that the city's goal is to come back to in-person learning as quickly as possible.

It's unclear when schools will reopen, but de Blasio has suggested the rate may not have to fall below 3% for in-person learning to resume. He also said that additional restrictions will likely be introduced this week by the governor's office, which will affect a number of different industries, including restaurants. 

Cuomo later addressed the varying messages between the city and the state in an appearance on WAMC shortly after the mayor's announcement.

“I set parameters and then I leave it to the school district with the caveat that they have to consult with the parents," Cuomo said. "Mayor de Blasio set 3%. He hit 3% today. He closed the schools at 3%. That is totally within his prerogative.”

He went on to say that he doesn’t think the decision to close schools was a mistake, but again suggested the mayor and school districts reevaluate their closing guidelines now that testing has been implemented in classrooms.

The infection rate in schools has remained remarkably low, below 1%, even as cases have jumped elsewhere in the city. 

The decision to return entirely to remote learning will put an even bigger burden on parents and students who don't have the necessary tools to take part in online instruction. According to Carranza, there are about 60,000 students in the school system who have indicated they are still in need of a personal device for remote learning.

"The issue continues to be the supply side," he said at the mayor's press briefing. "New York City along with every school system in America, probably the world, is ordering devices from every manufacturer."

About 40,000 devices are expected to arrive by the end of next week. In total, the city has ordered about 100,000 devices to address this need.

Free meals will continue to be provided at schools, which will stay closed at least through Thanksgiving, according to the mayor.