New York City's public school system, the nation's largest, will temporarily close beginning Thursday for in-person instruction because of increasing coronavirus cases in the city, school officials announced.

The city's 1.1 million students will now be taught entirely online, as most already are. To keep students spread out, the city offered in-person instruction only part time, with children logging on from home the rest of the time. At the end of October, only about 25 percent of students had gone to class in school this fall, far fewer than officials had expected.

The switch to all-remote instruction will disrupt the education of many of the roughly 300,000 children who have been attending in-person classes and create major child care problems for parents who count on their children being at school for at least part of the week.

The closure comes after schools have been open for in-person instruction for just under eight weeks. Mayor Bill de Blasio had warned of closing all school buildings once the positivity rate over a seven-day rolling average reached the threshold of 3 percent, and the city's health department said it reached that level on Wednesday.

Over the course of the pandemic, the department's numbers have often differed from the state, which said on Wednesday that the city had a seven-day rolling average of 2.5 percent.

The city was the center of the nation's COVID-19 epidemic in the spring, and its positive test rate dipped dramatically over the summer but has been gradually rising again in recent weeks.

School official said the spike in cases in the city doesn't appear to be caused by the reopening of school buildings, where they said virus transmission had remained very low since they reopened in September. As of midweek this week, more than 2,300 students or staff at the schools had tested positive since the start of the school year, city officials said.

New York City joins other large districts in cities like Boston and Detroit that have recently canceled in-person learning. Within the last week, the Clark County School District, which includes Las Vegas and is the fifth largest in the US, and Philadelphia's public school system both postponed plans to return to in-person learning.

Even as the school system stayed open, nearly 1,500 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.

The reopening of schools was originally set for Sept 10, but was postponed twice as teachers, principals and some parents said safety precautions and staffing were inadequate, with the teacher' union at one point threatening to strike. The city agreed to changes, including hiring thousands more teachers and testing 10 percent to 20 percent of all students and staffers per month for the virus.

Since October, New York state has been categorizing areas of high coronavirus risk with colors: red representing full restrictions, orange lesser restrictions and yellow indicates restrictions could be forthcoming.

Earlier Wednesday, Governor Andrew Cuomo warned New York City was on the verge of entering the orange zone of restrictions.

At a news conference, Cuomo said that he would shut down indoor dining in the city and impose other restrictions once the state's data showed that the city had reached a 3 percent test positivity rate over a seven-day rolling average.

Statewide, New York reported a seven-day average positivity test rate of 2.88 percent, and 2,202 people were hospitalized, Cuomo said.

Parts of the city where cases have risen in recent weeks have been subject to more restrictions, but officials have declined to impose restrictions across all five boroughs.

Cuomo said the positivity rate was 5.1 percent in parts of western New York. Parts of Erie County, which encompasses Buffalo, will be moved into an orange zone; other parts of the county and parts of neighboring Niagara County will become a yellow zone.

Agencies contributed to this story.

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