NEW YORK CITY (NewsNation Now) — New York City Mayor Bill de Blasio said on Wednesday that the city’s public school system, the largest in the U.S., would be closed for in-person learning starting on Thursday to ward off the increasing spread of COVID-19.

“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 (of) an abundance of caution. We must fight back the second wave of COVID-19,” the mayor said on Twitter. 

The school system said this decision would be until further notice.

As recently as Sunday, he said coronavirus test results had not reached the level that would trigger a shutdown of public school buildings.

De Blasio had previously warned that schools could close if the city crossed the threshold set earlier of 3% of coronavirus tests citywide coming back positive over a seven-day period. The rate stayed short of that on Sunday, at 2.57%, he said.

The city’s more than 1 million public school students will now be taught entirely online, as most already are. As of the end of October, only about 25% of students had gone to class in school this fall, far fewer than officials had expected.

In-person school resumed Sept. 21 for pre-kindergarteners and some special education students. Elementary schools opened Sept. 29 and high schools Oct. 1.

At the time, the seven-day positive test average rate was under 2%.

Even as the school system stayed open, more than 1,000 classrooms went through temporary closures after students or staffers tested positive, and officials began instituting local shutdowns in neighborhoods where coronavirus cases were rising rapidly.

New York City’s school system, like others across the nation, halted in-person learning in mid-March as the virus spiked.

While many big U.S. school districts later decided to start the fall term with online learning, de Blasio pushed for opening schoolhouse doors.

The reopening date, originally set for Sept. 10, was postponed twice as teachers, principals and some parents said safety precautions and staffing were inadequate, with the teachers’ union at one point threatening to strike.

The city agreed to changes, including hiring thousands more teachers and testing 10% to 20% of all students and staffers per month for the virus.