A bill to shrink class sizes in New York City schools has passed the state legislature — despite strong opposition from Mayor Eric Adams, who is expected to maintain accountability over the school system for the next two years.

The legislation, tied up with an extension of mayoral control during negotiations, limits the number of students per classroom over a phase-in period of five years.

It passed both the state Senate and state Assembly Thursday and into Friday. If signed into law by Gov. Kathy Hochul, kindergarten through third grade classes would be capped at 20 students; fourth through eighth grade at 23 students; and high school at 25 students.

“When we say small class sizes, I don’t even know if that’s the right term,” said State Sen. John Liu, who chairs the Committee on New York City Education, on the senate floor. “Because the plan — this legislation — calls for class sizes that are closer to the national norm, even closer to the rest of the state.”

“The class sizes in the city of New York are substantially larger than the rest of the state and the rest of the country,” he added.

The bills’ sponsors referenced studies showing students learn faster and perform better with fewer kids in the room — due to increased individualized attention, participation and communication between teacher and students.

But other research has shown that if inexperienced teachers are employed to staff the smaller classrooms, those gains are often canceled out, according to the education nonprofit Chalkbeat.

Adams and his schools chancellor, David Banks, have cast a specter over the measure, which they say will lead to unwelcome trade-offs and severe cuts throughout the school system.

“An unfunded mandate like this would potentially do huge damage to our system,” Banks warned on Wednesday night — citing a $500 million annual price tag for just elementary schools.

Liu pushed back on the notion that it lacks the proper funding, pointing to more than $1 billion in additional state cash through the Campaign for Fiscal Equity lawsuit. The bill passed his chamber in a landslide 59-4 vote.

The Citizens Budget Commission did not have a quick answer on costs either earlier this week, but warned that much of the research on the benefits of class sizes is below 20 students.

“It’s going to cost a lot of money,” said Andrew Rein, president of the nonprofit fiscal watchdog. “There’s not the evidence it’ll have a positive effect, and there’s the possibility it’ll have a negative effect elsewhere.”

Alongside a class size reduction plan and annual reports, the bill also requires the Department of Education to submit a financial impact statement in two years — that may recommend a pause of the class size reduction plan, but cannot undo progress made so far on class sizes.

The class size reduction measures come as the average number of students in New York City classrooms has already dwindled since the start of the pandemic.

“The fact that class sizes on average have already nearly reached the caps in the law, and that the DOE has five years to reach them citywide shows that achieving these goals will be affordable, given the political will to do so,” said Leonie Haimson of the advocacy group Class Size Matters.

“Nearly all the goals that the mayor has for our schools, including social emotional learning and his dyslexia initiative, are far easier to achieve with smaller classes,” Haimson added, “so that teachers can connect with their students more closely, provide them with the support they need, and screen and address any reading problems.”

The legislation includes exceptions for space restrictions or over-enrollment in school buildings — though the capital budget must demonstrate attempts to resolve those problems. It also creates carveouts in the cases of certified teacher shortages and “severe economic distress.”

Haimson told The Post she was concerned about the Adams administration and exemptions, vowing to “work to make sure that he doesn’t falsely claim that the city can’t afford this.”

The legislation could also benefit school staff, according to a fact sheet released by the United Federation of Teachers on Thursday.

“New York City suffers from high teacher attrition,” read the notice. “Roughly 5,000 instructors resign or retire every year, fed up with city teaching conditions — including oversized classes. The possibility of dramatically lowering class sizes could help retain many of these veterans.”

The teachers union also cited state figures that 663 of New York’s 675 public school districts have smaller class sizes than those of the city.

UFT lobbied for a similar measure last year at the City Council level, though the bill never came to a vote.

“This is a different proposal,” explained Sarita Subramanian at the Independent Budget Office, a publicly funded agency.

Under the local bill that was based on square footage, with a cap at about 18 students in an average-sized classroom — Subramanian said high schools faced the greatest challenges to accommodate students.

“If you have multiple classes at the same grade level, it is possible to switch students between classes,” she said. That becomes trickier if a school has a few children over the student limit interested in a chemistry or biology class.

“These are decisions that principals will have to make, and it’s unclear how that will work,” Subramanian said.

The state legislature also passed a controversial proposal to limit Adams’ control over the city schools to just two years before it is up again for renewal. The bills also expand the local school board called the Panel for Educational Policy to a 23-member body plus city officials.

“Today we have a bill that I don’t think anybody would say it’s perfect,” said Liu.

The mayor “will retain a tremendous amount of control over New York City public schools,” he added. “At the same time, we’ve heard our constituents, we’ve heard the parents of New York City school kids. And we’re making the system more responsive to them.”

Additional reporting by Bernadette Hogan.