NEW YORK (NYTIMES) - New York City's schools reopened on Monday (Sept 13) to scenes of joy, relief and trepidation, as roughly 1 million children returned to their classrooms, most of them for the first time since the country's largest school system closed in March 2020 because of the pandemic.

The day, always chaotic even in normal times, began with many families and educators anxious about what the next few months will hold, as the spread of the highly contagious delta variant has complicated the city's push to fully reopen schools.

The full reopening of the system represents a crucial moment in the city's long recovery from the pandemic, and Mayor Bill de Blasio has staked much of his legacy on keeping schools open, even as other districts across the country have faced large-scale quarantines and disruptions. In contrast to last year, and unlike some other major urban districts, the city did not offer a remote option to most students.

Yet it remains to be seen how many parents will decide to keep their children at home anyway - at least initially. Last year, 600,000 children were signed up for remote learning, and while the vast majority of those children seemed to have returned to schools Monday, a small group of parents have petitioned the city to resume online learning.

De Blasio has said that he expected some families to initially keep their children at home to see how the first weeks play out, but he said he believed they would all eventually return. It is not clear when attendance figures will be available.

Meisha Porter, the schools chancellor, said last week that the Administration for Children's Services could get involved if families refuse to send their children back after several weeks.

Speaking at a news conference on Monday, the mayor said that Monday would be remembered as "a day that was a game changer, a difference maker, a turnaround day" for New York City.

Most parents accepted that it was time to go back. In Brownsville, Brooklyn, Debra Gray nervously dropped off her 13-year-old son Kamari, who has asthma, at Public School 323. "We've got to give this a chance," she said. "The kids need time with their teachers. But I'm concerned."

To reassure parents that their children are returning to safe classrooms, city officials have implemented policies including random testing, vaccine mandates for school staff and quarantines for unvaccinated students.

But for all the planning, the online health screening survey that parents are required to fill out each morning temporarily crashed as hundreds of thousands of people logged on simultaneously.

Still, the day went off with few major hitches. Across the city, students expressed excitement and uncertainty about the new year.

On a subway car with broken air conditioning in East New York, Brooklyn, Neriyah Smith, who is 8, said she was nervous and excited about seeing her classmates again after learning remotely for all of last year. "I made a lot of friends before I was on computers," she said.

In the Bronx, Jazlynn Gonzalez, 14, hugged herself and stared wide-eyed at the students pouring into Herbert H. Lehman High School. "Ooh, I'm so scared," she said. "I don't know what to do, like people come up to me and I don't know if I should say hi, I just get confused."

New York, which always starts and ends its school year later than most other districts, is the last major system in the country to reopen.

Los Angeles and San Francisco have seen very few outbreaks in the weeks schools have been open, while other districts that do not require masks or other safety measures have seen mass student quarantines. In Mississippi, for example, which has one of the lowest vaccination rates in the country, there were 69 outbreaks at schools in the first few weeks of classes.

De Blasio has long said that the city, once an epicenter of the pandemic, would not be able to fully recover without the complete restoration of its school system, which will allow many parents to return to work. There are indeed encouraging signs: The city's delta wave, which was modest compared with much of the rest of the country, appears to be plateauing just as the school year begins.

De Blasio has said the city is not considering mandating shots for eligible children, as Los Angeles has done.

But New York has gone further than most districts in the country by implementing a full vaccine mandate for all its educators, along with all adults who work in school buildings.

The stakes are enormous for the hundreds of thousands of city children who have not seen their classmates and teachers since the start of the pandemic.

In the Bronx, Jazlynn said that her first-day-of-school jitters were about more than making the jump from middle school to high school: They were about relearning how to go to school. "I used to be very talkative to people, but now I keep my distance and I stay quiet now, that's what makes me more nervous," she said.

Nailah Frederick, a 15-year-old sophomore at Bayside, said she had been consistently receiving A grades for her work until the pandemic started.

"I can't learn online," she said, adding, "I didn't think my first year of high school would be like that. I've missed looking around a classroom and having people there around me."

The mayor has remained resolute that the school year will proceed normally, albeit with safety measures in place. But it is still possible that significant in-school transmission this fall could force many school buildings - or even the entire system - to shut down temporarily.

City schools saw remarkably low virus transmission in their buildings last year, but most schools were at significantly reduced capacity. Yet even with a low transmission rate at the end of last year, quarantines were still a regular occurrence.

The city's newly announced quarantine policy will almost certainly lead to frequent short-term classroom closures.

In elementary schools, where children are still too young to be vaccinated, one positive case in a classroom will prompt a 10-day quarantine, and a switch to remote learning, for that entire classroom.

In middle and high schools, only unvaccinated students will have to quarantine if exposed to someone with the virus, meaning that unvaccinated students could have a much different school year than their vaccinated classmates. Over 60% of New York City children eligible for the vaccine have received at least one dose, but the city does not know how many of those children attend its public schools.

Testing will begin this week. Asked on Monday about the city's testing protocols, de Blasio said that schools can ramp up testing if needed.

The city's modest testing program has made many educators uneasy, including the thousands of teachers who received medical waivers to work remotely last year. But Monday, all educators were back in school buildings.

Justin Chapura, who teaches English as a second language at Bronx River High School, said he was nervous and had trouble sleeping before school started. But he was overjoyed to see students he hadn't seen since March 2020 - some of whom had experienced major growth spurts.

"There's a million things going through my head: Do I have everything ready?" Chapura said. "Do I have all my copies made? What's my first class? What's my second class? Where's my lunch? What's happening? Do I have my coffee? I preordered my coffee in the taxi on the way here - nothing's going to screw me up today."