New York City Mayor Bill de Blasio announced Tuesday that America’s largest school system will delay in-class school instruction by 11 days to allow teachers and principals extra time to prepare to meet students amid the coronavirus pandemic.

In a deal struck with New York City’s powerful unions representing teachers, staff, and administrators, in-person instruction that was supposed to begin on Sept. 10 will be delayed until Sept. 16.

All students will begin the school year virtually learning from home, by computer, on Sept. 16 before a hybrid approach, involving some in-person instruction, begins five days later, de Blasio said, making the announcement alongside schools Chancellor Richard Carranza and United Federation of Teachers’ President Michael Mulgrew. Teachers will report as scheduled on Sept. 8.

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"One of the things that we affirmed in these discussions is that nothing, nothing replaces in-person learning," de Blasio said. "Our educators have been clear, that as important as it is to supply the very best remote learning that we can -- to address the digital divide, to provide free devices and internet services for kids, do everything we can to make remote work, nothing is as powerful as in-person learning."

De Blasio, a Democrat, explained that "mandatory" monthly COVID-19 testing will be carried out on a sample of students and staff in every school in the city's five boroughs through a  "medical monitoring program" conducted by health experts from both the mayoral administration as well as unions. Testing vans and tents will also be set up outside schools.

“We’re all working together to figure out the details. What I know for sure is that every single school will have testing. It will be done every single month. It will be rigorous. We are going to be looking constantly for any signs of a challenge we have to address,” the mayor said. “Anyone who tests positive will, of course, be isolated and it will trigger the test and trace apparatus.”

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Mulgrew, who heads the city’s largest teachers union, had for days threatened a strike if de Blasio pressed forward with plans to restart in-classroom learning Sept. 10. Tuesday morning’s deal was struck hours before an afternoon meeting by delegates of the United Federation of Teachers, which was poised to vote on whether to authorize the strike.

The United Federation of Teachers advocated for mandatory testing for adults and students and for the city to delay reopening until all schools were equipped with adequate masks and personal protective equipment, cleaning supplies, and improved ventilation.

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The development comes after de Blasio emphasized for months that the city’s 1.1 million public school students need schools to resume in-person instruction this fall after the coronavirus abruptly forced a thorny plunge into remote learning in March. New York City students had their last day of in-class instruction on March 13. All schools statewide were closed by March 18.

Gov. Andrew Cuomo said the state would allow a return to in-person school in regions where fewer than 5% of people tested for COVID-19 came back positive.

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The entire state has been well under that threshold all summer. The Democratic governor this month cleared schools to open around the state while cautioning that they still needed to address parents’ and teachers’ safety concerns.

The Associated Press contributed to this report.