Schools in New York City fully reopen after 18 months of pandemic restrictions.

Roughly a million children in New York City are returning to classrooms on Monday — most of them for the first time since the United States’ largest school system closed in March 2020.

While the city reopened schools last fall for part-time learning, the vast majority of students chose to keep learning remotely. But with no remote option now available to almost all parents, classrooms will be full for the first time in a year and a half.

For months, Mayor Bill de Blasio has forecast the first day of school to be a triumphant coda in New York City’s long recovery from the pandemic.

“This is going to be one of those game-changer days, one of those days we remember when we turn the corner on Covid,” the mayor said during a news conference last week.

But the spread of the highly contagious Delta variant has complicated the city’s push to fully reopen schools and left many families and educators anxious about what the next few months will hold.

In May, amid a brisk vaccine rollout and rapidly declining virus case counts, Mr. de Blasio announced that the city would no longer offer remote instruction to most students. (A few thousand children whom the city considers medically vulnerable will still be able to learn from home.) His announcement triggered little political resistance in the spring, but his administration has faced growing pressure from parents and politicians to reconsider.

About 600,000 families, most of them Black and Latino, kept their children learning from home last year. This year, while parents are much more receptive to reopening schools, some say they would like to wait at least until their young children are eligible for the vaccine. Only children 12 and older are currently eligible, and younger children may not be until later in the year, at the earliest.

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. Even with a transmission rate of 0.03 percent as of the end of last year, quarantines were still a regular occurrence.

This year, at least some level of disruption is inevitable.

Mr. de Blasio has acknowledged that he does not expect all children to actually return to school this week, since some parents have informed their principals that they want to wait a few days or even weeks to see how reopening goes.

A similar situation has already played out in Dallas, where some parents kept their children home for the start of school. Since then, students there have started to return to classrooms in higher numbers.

But 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.

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 percent 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.

While the city’s quarantine protocol is more conservative than the Centers for Disease Control and Prevention recommends, New York’s school testing plan is more modest than the C.D.C. calls for, alarming some parents and public health experts.

A random sample of 10 percent of unvaccinated students will be tested in each school every other week; the city was testing 20 percent of people in all school buildings weekly by the end of last year. Experts have said that the city’s current testing plan will almost certainly be too small in scope to stop many outbreaks before they start.

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 mayor has said he believes that the mandate, along with increasing vaccination rates for eligible students, will help keep schools as safe this year as they were last year.