NEW YORK (NYTIMES) - Of all the challenges that Mayor Bill de Blasio faced in his push to reopen the nation's largest school system in a pandemic, the city's powerful teachers union presented the most formidable obstacle, threatening an illegal strike as soon as this week.

But after a flurry of late-night negotiations, the mayor reached a deal early Tuesday (Sept 1) with unions representing teachers and principals, clearing the path for New York City to become the only major school district in America to welcome children back into classrooms this fall.

The city's 1.1 million schoolchildren will now start both remote and in-person classes on Sept 21, 10 days later than originally scheduled.

The new timeline gives educators more time to prepare for the country's most closely watched reopening effort, and provides the mayor with a longer runway to pull off one of the most ambitious, and riskiest, city initiatives in decades.

"For the nation's largest school system to come together in unity and say, we are going to get it right, and it won't always be easy and there'll be tough moments along the way, but we're going to get it right - that's a statement," Mr de Blasio said during a morning news conference.

Seated beside the mayor was Michael Mulgrew, the influential leader of the United Federation of Teachers, who just a few hours earlier had been publicly warning of a strike.

With a deal in place, Mr Mulgrew considerably shifted his tone, declaring "that the New York City public schools system has the most aggressive policies and safeguards of any school system in America."

But Tuesday's triumphant announcement did not obscure the difficulties Mr de Blasio and the unions still face.

While the city has made progress in distributing personal protective equipment to schools, hiring nurses for every school building and upgrading ventilation systems in classrooms, that work is far from complete.

And even with the delay, principals will need to scramble to address the staffing issues necessary to create two complementary versions of school, one in-person and one online.

Now that the city has agreed to a number of union demands, the onus will fall again on the city's educators to actually make the reopening plan work. Many principals have spent the last few weeks inside their school buildings, preparing for students.

The announcement followed weeks of escalating pressure from union leaders, elected officials, teachers and principals who said they would not be ready to reopen as planned on Sept 10.

The city's teachers union, which has not staged a strike in nearly half a century, was poised to authorise its 75,000 members to walk off the job.

Agreeing to a major union demand, Mr de Blasio said the city would require monthly, random testing of between 10 per cent and 20 per cent of students and staff in all city school buildings starting in October, with results ready within 48 hours. Students and staff members who refuse to get tested will not be allowed in school buildings.

Teachers will report to schools as scheduled next week, and will start meeting with children virtually to review health and safety rules and ensure that all their students have devices for remote learning.

The deal was announced about a week before school was scheduled to begin, leaving working families already desperate for clarity about schools with little time to rearrange their schedules.

Still, if the mayor succeeds in bringing many of New York's children back into classrooms, he will stand apart from every other big-city mayor in America, all of whom have opted to start the school year remote-only, as the virus has surged in their cities and political opposition to reopening has intensified.

Earlier this summer, large urban school districts from Los Angeles and Chicago reversed their plans to offer in-person instruction, and said they would re-evaluate when the virus was better contained in their cities. Scores of rural and suburban districts have also started the year with fully remote classes. By some estimates, half of America's children will spend a large chunk of the fall learning remotely.

The stakes of the reopening endeavour in New York pushed the mayor and union leaders to start negotiating in earnest over the weekend, with the school start date and testing protocol for students and staff left as the two major outstanding issues.

City Hall staff members exchanged e-mails, some with 60 responses each, between 4.30am and 7am on Tuesday. At around 8am, the mayor called the union leaders and asked them to come to City Hall for a final meeting.

The frenzy of late-night talks were in keeping with the last few frantic weeks of preparation, and understandably so: The push to reopen schools, which is crucial for New York City's economic recovery, has been an enormous effort.

The schools chancellor, Richard A. Carranza, began a recent memo to principals with a reminder of the tremendous pressure that the administration was under. "There are many things that keep me up at night," he wrote. Carranza had begun a virtual meeting with parents the other evening at 6pm and finished just before 4am.

The mayor pushed his team by arguing that the system's mostly low-income, Black and Latino students urgently needed in-person classes, an assertion widely supported by education experts.

New York has an enormous population of vulnerable public schoolchildren who have been largely failed by remote learning: About 750,000 public schoolchildren in New York City are poor, roughly 200,000 have disabilities and 114,000 are homeless.

But the mayor's insistence that schools would be ready to reopen as originally scheduled on Sept. 10 frustrated many teachers and principals, who said they did not believe de Blasio understood the depth of the challenges they faced on the ground.

Officials have vowed to distribute 4 million face masks, 3.5 million bottles of hand sanitizer and 80,000 containers of disinfectant wipes. More than 3,500 electrostatic sprayers - special equipment that has been used on the subway - are being deployed to disinfect surfaces.

But getting personal protective gear and sufficient soap and hand sanitiser into the city's public school buildings is only a first step.

Teachers raised concerns about ventilation in aging school buildings and said they did not understand how often they or their students should be tested.

Beyond safety concerns, educators said they did not know how many children would actually report to school, or how many staff they would have to teach students in the building and online.

Under the mayor's plan, most children will report to school between one and three days a week, and have online classes the other days. They will return to classrooms that have been transformed since they left suddenly in March.

Desks will be spaced 6 feet apart, so most classes will have only nine or 10 children at a time, about a third of the typical capacity. Students, teachers and other staff will be required to wear masks all day, except for a quick lunch period held in classrooms.

Many hallways will be marked with signs indicating where students should line up to maintain distance in hallways and bathrooms. Windows will be open, even during cold and rainy days, to allow for more fresh air.

The research on school reopening in places with similarly low virus transmission rates largely bolsters the mayor's decision to welcome children back into classrooms. Research from Europe has shown that schools can successfully open as long as the virus is contained in their region and families understand there will be some unavoidable turbulence.

New York City has so far defied predictions of a second wave of the outbreak. In recent days, the positivity rate for virus tests has hovered around only 0.6 per cent, far lower than the national average. Mr de Blasio has said schools will not reopen, or will automatically close, if the citywide average positivity rate reaches 3 per cent.

Public health experts generally agree that the city's schools can reopen, as long as adequate testing and strict safety measures are in place.

White parents are more willing to send their children back into classrooms than parents of colour, according to a recent poll conducted by The Education Trust, a research group. About 34% of city parents have already decided to keep their children at home full time, and that number is almost certain to climb over the next few weeks.

Still, more than 600,000 families are tentatively planning to send their children back into schools later this month.

"Having all of the major parties come to the table and work out an agreement that will help New York's children and families is a big deal," said Rebecca Kirszner Katz, a parent of two public school parents and a former mayoral adviser who has frequently criticized the mayor this year.

"De Blasio has become a punching bag for months, but this is arguably one of his best moments, and now we just have to hope this works."