New York Gov. Kathy Hochul signed a bill Thursday that will require New York City to reduce the class sizes, approving the measure in exchange for state lawmakers agreeing to delay it by a year.

The agreement means the city will soon have to cap classes at 20 to 25 students, depending on the grade level. The change will be implemented gradually over a five-year period beginning next year and ending in September 2028.

“This bill reflects the need to ensure students have dedicated teachers devoting time and attention to their learning in person as we continue to come back from the COVID-19 pandemic,” Hochul wrote in her approval message.

State lawmakers approved the bill in June alongside an extension of mayoral control of the city school system. The five-year phase-in was originally supposed to begin this month.

But Mayor Eric Adams pushed back against the measure, arguing that it would cost the city millions of dollars to hire more teachers and secure more classroom space to account for smaller class sizes. He called on Hochul and state lawmakers to come up with funding to implement the change.

In the end, Hochul agreed to sign the measure on the condition that it doesn’t begin to take effect until next September. State lawmakers are expected to approve a chapter amendment — a tweak to the bill — when they return to the Capitol in January.

Despite the tweaks to the bill, Albany won’t be providing a funding source to implement the changes. Instead, the law makes a vague promise to take into account the cost of such changes when the state budget is negotiated in the spring.

Once fully implemented, the soon-to-be new law will cap kindergarten to third-grade classes at no more than 20 students, while classes for grades four through eight will be limited to 23 students. High school classes will be capped at 25 students, while physical education and performing arts classes will be allowed to have up to 40 students per class.

During the five-year phase-in period, an additional 20% of classes will have to be in compliance with the rules each year, according to the bill. The city will have to prioritize classes in higher-poverty areas first.

Under the current rules, city classes are limited to 32 to 34 students in most cases.

Hochul’s office had been negotiating potential changes with lawmakers and the mayor’s office in the months since the bill passed. Late last month, Hochul said on WNYC’s "Brian Lehrer Show" that one of the things under consideration was a potential “funding source.”

But in the end, the only pending change Hochul cited in her approval memo was a one-year delay in implementing the bill.

“In doing this, the amended legislation directs the parties to commence the plan development process immediately in order to prepare for implementation beginning in September 2023,” she wrote.

The bill does not specify any guaranteed additional funding for the city, though it does require the state and city comptroller’s to review the city’s plan for implementing the class-size cap and identify what kind of money it would require. The law says the state will have to take that into consideration when it determines how to dole out education funding in the state budget process each year.

Michael Mulgrew, president of the United Federation of Teachers, praised Hochul for signing the bill into law. The union had been a major driving force behind the class-size legislation, despite Adams’ concerns.

In an interview, Mulgrew called the deal “monumental” and said he had no problem with giving the city an extra year to implement the smaller classes.

He said that with the signing of the bill, Hochul had put “herself in very high esteem with the parents, students and teachers of New York City.”

The deal marks a huge victory for the UFT, an influential union, that has butted heads with Mayor Adams on the issue of smaller class sizes as well as cuts to the city’s education budget.

Mulgrew maintained that the city's education department — which has has skirted a court-ordered mandate for smaller class sizes for years by arguing that the city cannot to afford to fund such a policy — could not ignore the legislation.

He added that any attempt by the Adams to protest the mandate would be received poorly by the public.

“I would say politically, ‘good luck,’” he said. “This is the number one issue with all the parents of New York City.”