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NEW YORK (AP) — New York City postponed the start of its school year by several days to allow more preparation to reopen classrooms amid the coronavirus pandemic, announcing a delay Tuesday after teachers said they might OK a strike over the city’s drive to open schools.

The change comes nine days before the nation’s largest public school system was set to bring students back to classrooms.

Now, instead of starting a mix of in-person and remote learning on Sept. 10, the city’s more than 1 million public school students will start remote-only classes Sept. 16. In-person instruction will begin Sept. 21, though over 360,000 students’ families have opted to stick with remote-only learning altogether.

“It is a revision that still allows us to keep things moving forward on a tight timeline, but with additional preparation time,” Mayor Bill de Blasio said.

The delay is meant to allow for finishing virus-safety steps that were already planned and working out some new provisions — including random virus testing of 10% to 20% of all students staffers per month, in an effort to get a handle on whether people without symptoms might be in schools and potentially infectious.

City officials had emphasized for weeks that schools were making extensive preparations and would reopen safely Sept. 10, but school staffers said the city wasn’t doing enough. The teachers’ union, the United Federation of Teachers, said Mondayit was negotiating with the city but could authorize a strike vote if no deal was reached by Tuesday afternoon, even though New York state bars teachers and other public employees from striking.

By Tuesday morning, the heads of the teachers’ and other school unions joined de Blasio to herald what he called an agreement “to address real concerns that have been raised about how to do things the right way, how to do them the safe, healthy way.”

United Federation of Teachers President Michael Mulgrew said the new plan represented “the most aggressive policies and greatest safeguards of any school system in the United States of America,” he said.

Still, some teachers said more needed to be done to ensure safety.

“A 10-day delay doesn’t do that. Random testing doesn’t do that,” said Madeline Borrelli, a city public school teacher and parent who gathered with some other teachers at a demonstration in Brooklyn on Tuesday. “What we need is a delay, fully remote.”

The postponement comes after the mayor, a Democrat, emphasized for months that the city’s pandemic recovery depends on getting students back in classrooms in person after the coronavirus abruptly forced a thorny plunge into remote learning in March.

New York suffered the deadliest coronavirus spike in the country before getting the virus’ spread in check in late spring, even as it climbed anew in many other parts of the country.

Even as other big U.S. school systems — including Chicago, Los Angeles, Miami and Houston — decided to start the school year with students learning remotely, de Blasio stuck with the city’s plan for a hybrid reopening. It calls for students physically going to school part-time, on staggered schedules, and learning from home the rest of the time.

The mayor said Tuesday he empathized with parents who have been planning on sending their children back to school Sept. 10, but he cast the delay as “a very modest change to resolve outstanding issues so we can all move forward together.”

He said he believed the new scheduled allowed enough time to resolve safety concerns, and union leaders also sounded optimistic notes. While there is plenty of ironing-out to do, “there is no longer a disagreement about what a school needs to have” to open and operate, Mulgrew said.

The city’s plan to restart schools already included mask-wearing, staggered schedules to reduce the number of students in rooms, supplying every school building with a nurse and asking all staffers to get tested shortly before school starts. The city dispatched ventilation experts to check out air flow in classrooms, and officials said they would work to make parks and streets available as teaching space if principals were interested.

The teachers’ union had pushed for requiring testing — either for active cases of the virus or for antibodies to it — for all students and staffers who plan to be in school.

Representing a compromise, the random-sample plan will vary by school in the size of the sample, but parental consent will be required for children to participate.

New York City students had their last day of in-class instruction March 13. All schools statewide were closed by March 18.

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Associated Press video journalist Ted Shaffrey contributed.