NEW YORK (CBSNewYork) — The remainder of New York City public school children headed back to in-person learning Thursday.

It happened amid a recent sharp increase in coronavirus cases in several neighborhoods.

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As CBS2’s John Dias reported, the city is still marching forward with its plan to get everyone who chose blended learning back into the classroom. But it has said if the number of positive cases continues to trend upward, schools may have to shut down. The daily number city-wide recently rose above 3%, but that has not been the case with the seven-day average, officials said.

United Federation of Teachers President Michael Mulgrew said if the city doesn’t get a handle on COVID clusters he will demand schools in those areas close.

“If those numbers don’t come down, this is where it’s going to land up in a bit of a fight, because if they can’t get those numbers down — I know that the mayor is continuing to say it’s 3% and it’s the entire city, all or none — that does not work for us,” Mulgrew said.

“All we ask is whatever decisions are made, are not based on politics, they are not based on economics, but they are based on preserving lives,” teacher Curtis Buckner added.

Schools: The New Normal

Mayor Bill de Blasio said he is sending more resources to the now 10 zip codes with rising cases and passing out violations, but added time will tell if he shuts down schools.

“We are watching and the decision will be made based on the facts on whether we need to do a fuller shutdown in those communities. But so far, again, we are seeing a real separation between what is happening in some of those neighborhoods and what is happening in the public schools nearby,” de Blasio said.

Despite all that, the mayor seemed confident, calling the now-1,600 schools open “extraordinary.”

Thursday marked the last phase of the city’s three-part reopening plan. The types of schools that reopened included middle, high and secondary schools, as well as transfer schools, adult education and evening schools.

Earlier this week, elementary schools started up again.

“A lot of people said it couldn’t be done and it was tough, but we did it,” de Blasio said.

Some students said they are thankful.

“It’s my senior year and I wanted it to live up to that,” said Shaxlo Bakhritdinova, who attends University Neighborhood High School.

Bakhritdinova was one of the first in a socially distanced line outside her Lower East Side school. With traditional proms and graduations still up in the air, she didn’t want to miss out on anything else.

“I’m not about that. It’s not cool. Everything is postponed or to be decided,” she said.

Other students said they hope their peers act responsibly.

“Just depends about, do they care about virus? Do they care about the pandemic. Do they care about the lives?” Jacob Tamares said.

“I don’t know what’s going to go forward from now,” Joshua Nunez added.

Watch John Dias’ report —

The first day of classes didn’t go off without a hitch.

Classes at DeWitt Clinton High School in the Bronx are supposed to start at 8:15 a.m., but the need to do temperature checks delayed everything. Some kids were an hour late getting into the school.

“It was pretty long lines at first. I came here around 7:20 a.m., then I had to walk over there to do temperature. It was a lot,” student Kayla Wiltshire told CBS2’s Marcia Kramer.

But that wasn’t the only issue as New York City embarked on getting its 1,600 schools open for in-person learning. A teacher shortage loomed large.

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While the mayor and Schools Chancellor Richard Carranza excitedly elbow-bumped returning middle school students in the Bronx, Mulgrew said the city still needed many more teachers.

“At this point, we’re assuming we need a couple of thousand teachers, which is why, especially at the high school level, we’re seeing some high schools that have very limited programs,” Mulgrew said.

That may be why the Department of Education sent a letter to substitute teachers offering a special financial incentive for those willing to teach in-person through the end of the month.

The city is offering an additional $100 a day over the regular daily rate for up to five days of in-person service and an additional $500 if the teacher works 15 or more days.

“It takes weeks in a normal school year for all the staffing realities to sort out. It will take weeks here, too,” de Blasio said.

WATCH: United Federation Of Teachers On First Day Of High School 

In addition, parents reported problems for those who chose to learn remotely at home and also started Thursday.

Daniel Bernstein’s son is an 11th grader in a Carroll Gardens school.

“One of the biggest issues here is that there’s no central location where their schedules can be found along with class times and Zoom links,” Bernstein said.

There was also another hiccup — New York City high school teacher Naun Umanzor tweeted video of an empty classroom to show his entire class failed to show up.

“The kids did have to get temperature checks, so maybe that delayed them coming in on time,” Umanzor, who teaches special education, told CBS2’s Jessica Layton.

It’s a kink that will need to be worked out because screening for symptoms is more important than ever.

“I understand the clusters can definitely scare people,” Umanzor said. “The state and the city I know are trying to contain that.”

The mayor refused to give details about attendance.

“We want to get these numbers right. It’s going to take more time,” de Blasio said.

At DeWitt Clinton High School, as with many high schools across the city, many students still had to learn remotely, even though they were sitting in classrooms.

“This is my first year of being back in school with it like this, so it’s like, yeah, kind of weird,” ninth grader Fiona Francis said.

Ninth grader Kayla Wiltshire says she did all of her classes remotely.

“I did gym, human bodies, global, Spanish, math, all of them,” she said.

Still, ninth grader Senaiya Edwards says it’s “way better” than being home.

“We’re going to continue to get better,” Carranza said.

The mayor says things went so well, he expects more students to opt into in-person learning. He touted Thursday as another step on the city’s road to recovery.

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