Classroom doors swing open for about a million New York City public school students on Monday in the nation’s largest experiment of in-person learning during the coronavirus pandemic.

The start of the school year coincides with several milestones in the city’s pandemic recovery that hinge on vaccine mandates.

Nearly all of the city’s 300,000 employees will be required to be back in their workplaces, in person, Monday as the city ends remote work. Most will either need to be vaccinated or undergo weekly COVID-19 testing to remain in their jobs.

The city was also set to start enforcing rules requiring workers and patrons to be vaccinated to go indoors at restaurants, museums, gyms and entertainment venues. The vaccination requirement has been in place for weeks, but had not previously been enforced.

There will also be a vaccine mandate — with no test-out option — for teachers, though they have been given until Sept. 27 to get their first shot.

Unlike some school districts across the country that are still offering online instruction to families that prefer it, New York City officials say there will be no remote option despite the persistence of the highly transmissible delta variant of COVID-19.

New York City kept schools open for most of the last school year, with some students doing a mix of remote and in-person instruction, but the majority of families chose all-remote learning. That choice won’t be available this year, Mayor Bill de Blasio has insisted.

“Our kids need to be in school and it’s unbelievable that some kids haven’t seen the inside of a classroom for a year and a half,” the mayor said Thursday. “There are massive consequences to that, including health care consequences. The healthiest, best place for kids to be is in school.”

Masks will be required for all students and staff members, as is the case in schools across New York state.

There is no vaccine mandate for students 12 and over who are eligible for inoculations, but vaccinations will be required to participate in contact sports like football and basketball as well as some extracurricular activities like band practice and theater. About two-thirds of the city’s 12-to-17-year-olds are currently vaccinated.

In the U.S., anyone 12 and older is eligible for COVID-19 vaccines. The Food and Drug Administration’s vaccine chief said last week he is hopeful children as young as 5 will be eligible to get vaccinated by the end of 2021.

De Blasio, a Democrat in his final months in office, has insisted that masks, cleaning protocols and random COVID-19 testing makes school buildings safe. But he has gotten pushback both from parents who want their children home and from unions representing teachers and other school staff members.

One caller to WNYC during the mayor’s weekly radio appearance Friday said she was “absolutely beside myself with fear of sending my 6-year-old into school.”

“We believe this is an extraordinarily safe environment,” de Blasio responded. “We’ve proven it and the most important thing is our kids have to come back.”

Asked if some students might just disappear from the system because their virus-wary parents won’t send them to school, de Blasio said “the vast, vast majority” of parents would bring their children to school.

The city has been in arbitration with the United Federation of Teachers, which represents almost 80,000 teachers in city public schools, over issues including accommodations for teachers who say they have health issues that prevent them from being vaccinated.

The arbitrator ruled late Friday that the city must offer non-classroom assignments to teachers who aren’t vaccinated because of medical and religious exemptions.

“As a group, teachers have overwhelmingly supported the vaccine, but we have members with medical conditions or other reasons for declining vaccination,” UFT president Michael Mulgrew said in a news release.

Meanwhile, other unions for city workers have objected to the mayor’s decision to order employees back into workplaces, saying that if they were performing their jobs well remotely, they should be allowed to continue.

The Municipal Labor Committee, an umbrella group for unions representing municipal workers, has also threatened legal action if the mayor moves to eliminate the option of weekly virus testing for workers who opt not to get vaccinated.

And a group of restaurant and bar owners has sued over the vaccination requirement for indoor dining and employees, saying the city has overstepped its legal authority.