NEW YORK (CBSNewYork) — As New York City teachers headed back to work Thursday to prepare their classrooms for the start of school next week, Mayor Bill de Blasio continued his push to get everyone vaccinated by launching a program offering shots at city schools.

The mayor also expanded the COVID vaccine mandate to include more education workers, CBS2’s John Dias reported.

READ MORE: NYC Teacher, Principal Unions Warn Of School Staffing Shortages When Vaccine Mandate Takes Effect; De Blasio Says Substitutes Standing By

Success Academy Midtown West started in-person learning more than a month ago. But come Monday, the city’s official first day for public schools, hundreds more will fill the building that’s shared by four institutions.

“I’m a bit concerned because it’s going to be more crowded,” said Arena Aziz, a parent.

WATCH: Mayor de Blasio’s Thursday, Sept. 9, 2021 COVID Update

With no remote learning option, it’s a prime example of what those in the education realm could soon face: capacity problems, especially in common areas like cafeterias.

Classrooms will look different than last year, too. Students will still have to maintain 3 feet of distance.

“Give us bigger buildings. Give us bigger classrooms,” said Ivette Dobarganes, a Spanish teacher at Stephen T. Mather High School, who knows her dream solutions are unrealistic.

CORONAVIRUS PANDEMIC

Universal masking, disinfecting and ensuring fresh air in classrooms and common areas are also part of the safety requirements.

“This year, is a greater challenge for the entire city,” said UFT President Michael Mulgrew. “We’re going to go from basically 300,000 students to almost a million students.”

Which is why Mulgrew is arming 75 trained staff members with carbon dioxide monitors to test the ventilation of each school in case someone feels there’s an issue.

“This is the gold standard to check,” Mulgrew said.

Watch John Dias’ Report

As CBS2’s Jessica Moore reported, the mayor brought out the big guns to stress the importance of his 100% in-person learning policy for all city schools.

“In-person learning is so important for children and teens. It’s important to their education, but it’s also important for… mental health and nutrition and exercise, all these other pieces,” said Dr. Lee Beers of the American Academy of Pediatrics.

A handful of city officials continue to push for a remote learning option, but the mayor isn’t budging. The only time it would happen is when a classroom shuts down after a positive test or when a student has to quarantine because of COVID-19 exposure.

READ MORE: Photos Of Lunches Served At Paterson Schools Show Mystery Meats, Puny Portions

The city also announced its “Vax to School” campaign, where students 12 and older can get the shot at 700 schools during the first week of classes with parental permission.

Currently, 65% of 12 to 17-year-olds in the city have received at least one dose. The mayor stopped short of following cities like Los Angeles in requiring students 12 and older be vaccinated.

“If there’s a family that’s not yet ready, I don’t want that family kept out of school. I want them in school where we can help their kid, especially for the kids who’ve been out for the last year and a half,” de Blasio said.

COVID VACCINE

The mayor also announced employees at all city-contracted sites must be vaccinated, including 3-K, after school and home care programs.

He’s sticking with his zero tolerance policy toward teachers who don’t get vaccinated, saying those who refuse will be removed from the payroll.

“The position that they took was, basically punishing people who have done a great job, and not really recognizing that people do have medical conditions that really require accommodations and those things are what we’re working out right now,” said Mulgrew.

While the city is currently in arbitration with the union over the vaccine policy, the mayor said very few medical or religious exemptions will be honored.

“He cannot make that statement. We have no idea,” said Mulgrew.

The city said 72% of teachers are currently vaccinated, but the union says the number is actually closer to 85%.

The union vowed to continue encouraging all teachers to get the shot by the Sept. 27 deadline.

CBS2’s John Dias contributed to this report.

Yonkers Goes Back To School

Thursday is the first day of classes for public school students in Yonkers. Excited school leaders greeted children arriving at Kahlil Gibrian School.

Temperatures were taken as students entered the building. Everyone is required to wear face coverings.

CDC guidelines, like placing students at least 3 feet apart, are enforced in classrooms.

MORE NEWS: 'I Want A Proper Education': Some NYC Public School Students With Medical Exemptions From In-Person Instruction Feel They're Falling Behind

Employees must either get vaccinated or be tested for COVID twice a week.

Jessica Moore