Nearly 1 million New York City students headed back to the classroom Monday morning — but the first day of school hit a snag when the city Department of Education’s health screening website crashed.

The screening on the website, which teachers and students are required to complete each day before entering the building, refused to load, or crawled for some, before the first bell. It was back up just before 9 a.m.

“The DOE Health Screening tool is back online. Our apologies for the short period it was down this morning. If you are having issues accessing the online tool, please use a paper form or inform school staff verbally,” NYC Public Schools tweeted.

Mayor Bill de Blasio addressed the glitch, telling reporters, “First day of school, a million kids, that will overload things.”

At his daily press briefing later in the morning, Hizzoner couldn’t say why the site had crashed.

At PS 51 in Hell’s Kitchen, staffers were getting parents to fill out paper copies of the health screening as their children waited in line to enter.  

For many students, Monday is the first time they have been back in a classroom in 18 months after the COVID-19 pandemic shut down the nation’s largest school system in March 2020.

De Blasio gave students celebratory fist bumps on their first day back at PS 25 in the Bronx.

“We want our kids back in school, we need our kids back in school. That’s the bottom line,” the mayor said outside the school.

“We need parents to understand if you walk into a school building, everything’s cleaned, the ventilation is taken care of, everyone’s wearing a mask, all the adults are going to be vaccinated,” he added. “It’s a safe place to be.”

Schools Chancellor Meisha Porter conceded there were still students being kept home because their parents are worried about the highly contagious bug, which is seeing a resurgence in cases across the nation due to the Delta variant.

Official enrollment figures for the 2021-22 school year have not yet been collated, with de Blasio saying it would take days to figure out. 

“We understand the hesitation and fear. It’s been a really rough 18 months, but we all agree the best learning happens when teachers and students are together in classrooms,” she said.

“We have the vaccine, which we didn’t have a year ago, but we’re prepared to increase testing if we have to.”

She hailed Monday as a “tremendous day for all of New York City.”

“I am the biggest cheerleader for this system,” she said.

De Blasio has been touting the return to classrooms for months, but the spread of the Delta variant has led to an array of issues ahead of the reopening, including concerns over vaccinations, social distancing and a lack of remote learning.  

Angie Bastin, who dropped her 12-year-old son off at Brooklyn’s Erasmus school Monday, told The Post she had fears regarding COVID.

“The COVID is coming back, we don’t know what’s going to happen. I’m very worried,” she said.

“I’m nervous because we don’t know what’s going to happen. They’re kids. They’re not going to follow all the rules. They’re going to eat, they’re not going to talk without masks. I don’t think they’re going to follow the rules they tell them over and over, because they’re kids.”

Meanwhile, Dee Siddons — whose daughter is in eighth grade at the school — said while she too was concerned about COVID, she was happy for her kids to get back in the classroom.

“I’m excited that they’re going back to school. It’s better for their social and mental health, and their social skills, and I’m not a teacher, so I’m not the best at home, but it’s a little nerve-wracking,” she said.

“I am concerned about them taking precautions, but you have to teach your kids the best way to take care of themselves, because I can’t take care of anyone else’s kids.”

There is no vaccine mandate for students over the age of 12 who are eligible to receive the shot. About two-thirds of students ages 12 to 17 have already been vaccinated, according to the city.

But teachers are required to be vaccinated — and they have been given until Sept. 27 to get their first dose.

The directive has proven challenging, with 36,000 Department of Education workers — including more than 15,000 teachers — remaining unvaccinated as of last week.

The United Federation of Teachers has been fighting portions of the mandate and scored a win against the city last week when an arbitrator ruled the city needs to provide accommodations to DOE staff with medical conditions or religious beliefs that preclude them from getting a COVID-19 vaccine.

Greeting teachers at PS 51 in Hell’s Kitchen first thing Monday, UFT president Michael Mulgrew lauded returning staffers for their efforts in helping to reopen the school system.

But he also acknowledged a slew of unresolved concerns.   

Mulgrew said he hoped last week’s ruling on the fate of unvaccinated teachers would lead to a surge in shots — but admitted the city could conceivably lose thousands of educators.

“It’s been a real challenge,” Mulgrew said of trying to soothe tensions related to the vaccine.

The city has also faced pushback from some parents who want to keep their children home.

Unlike last year, New York City officials say all-remote learning won’t be an option this school year.

The city kept schools open for most of the last school year, with some students doing a mix of in-person and remote learning. The majority of parents chose all-remote learning.

Students who are quarantining due to COVID-related illnesses or are granted medical exemptions will be allowed to learn remotely. Those who are vaccinated and asymptomatic won’t have to quarantine if there’s a positive COVID case in their classroom.

Mom of four Stephanie Cruz grudgingly waved her children off to school at PS 25 in the Bronx, telling The Post she would have preferred to keep them home.

“I was a little bit nervous and scared because the pandemic is still happening and my kids are going to school,” Cruz said.

“I am nervous about my kids keeping their mask on during the day and staying safe. I was hesitant to send them off. 

“I will be ecstatic when my kids get home safe and I can’t wait to hear about their first day.”

Among the protocols the city is enforcing for the reopening are mandatory masks for students and staffers, 3 feet of social distancing and upgraded ventilation systems.

The city’s principals union — the Council of School Supervisors and Administrators — has already warned that many buildings will lack the space to enforce the 3-foot rule.  

Jamillah Alexander, whose daughter is in kindergarten at PS 316 Elijah School in Brooklyn’s Crown Heights, said she was concerned about elements of the new COVID protocols.

“They’re not going to shut down unless it’s two to four cases. It used to be one. It was 6 feet of space, now it’s 3,” she said.

Other parents said they had warned their children not to take off their masks.

“I told her to keep her mask on at all times. You can socialize but don’t get too close to anyone,” Casandria Burrell told her 8-year-old daughter.

Several parents dropping their children off at PS 118 in Brooklyn’s Park Slope were frustrated by the school’s requirement for students to bring their own supplies, including disinfectant wipes and even printer paper.

Tia Elgart said the supplies set her back $250 for her two children.

“It was last minute and it wasn’t optional,” she said.

“I guess we’re supplementing the budget. They lost a lot of students last year so they’re hurting financially and the standards for these parents are very high.”

As Whitney Radia dropped her 9-year-old daughter off at school, she also noted the high cost of providing school supplies.

“They had to bring their own everything. No shared school supplies,” Radia said.

“At least $100 per kid, more to be honest. The normal stuff like notebooks, folders and pens but also baby wipes, tissues, paper towels, their own scissors, markers, colored pencil sets, printer paper. The stuff that used to be communal.”

Additional reporting Jackie Salo and Kevin Sheehan