The Mamdani administration plans to spend more than $1 billion to hire new teachers and construct new space at school buildings to comply with New York’s new class size law, even as overall enrollment in the public school system drops.

Mamdani pledged in a new plan released Thursday to include $244 million in this year’s budget to hire teachers, as well as $1.5 billion in capital funds for new space.

The city has already spent $450 million on additional teachers to meet the mandate. Mamdani said the investment demonstrates his administration’s support of the class size law. His predecessor, Mayor Eric Adams, had criticized the law, citing the cost.

“For too long, City Hall treated class size as a problem to postpone rather than a promise to keep,” Mamdani said in a statement. “That ends now.”

Some advocates for smaller class sizes criticized the new plan as insufficient.

Leonie Haimson, executive director of Class Size Matters and leading champion of the law, called the plan vague.

“The class size reduction plan released by DOE today is hugely disappointing and is not a real plan at all,” she said.

She said the plan includes “boilerplate language” and “fluff” rather than “concrete strategies” and details about where and how schools will gain space.

In addition to construction, Haimson has proposed capping enrollment at overcrowded schools to shrink classes and strengthen under-enrolled schools nearby, boosting their budgets so they can offer more arts and electives. She said limiting enrollment is a quick, less costly option compared to construction that can take years and cost billions of dollars.

“They will never get there without aligning their enrollment policies,” she said.

The new report calls reducing enrollment at popular schools “a lever that the parties crafting this plan have discussed and would prefer not to utilize unless it is necessary.”

Officials have said lowering access to those popular schools could push families out of the public school system altogether.

Some popular schools, including all the specialized high schools, will continue to receive exemptions from the class size law.

Schools that are just slightly over their requirements may see enrollment adjusted a little, by allowing fewer students to enroll midyear, the plan says.

Mamdani has said he’s a strong supporter of the effort to reduce class sizes, noting that he voted for the law in the state Assembly.

The 2022 law, which is being phased in, calls for no more than 20 to 25 students in classes depending on grade. Many parents and teachers have fought for decades for smaller classes, citing better academic and social outcomes for kids. But it is being rolled out as the city’s public schools have lost some 160,000 students since 2020, a combination of lower birthrates and families leaving the city. Experts expect that trend to continue. The school system’s construction division projects the city will lose almost that many students again over the next decade.

According to the new plan, the city will have to hire 10,000 to 12,000 more teachers and build more space at hundreds of schools to reduce class sizes. Hundreds of schools are slated for new construction.

Some fiscal watchdogs have criticized city leaders for planning new construction at popular schools when the school system is losing students.

“Building new space for a school system that is shrinking in enrollment in a city that faces fiscal challenges is not the right approach, and it is not an effective use of our scarce resources,” said Ana Champeny, vice president for research at the Citizens Budget Commission.

The plan also offered more insight into the areas where classes are most crowded, showing that schools with the poorest families are most likely to already be meeting class size targets, while schools with more affluent families are more likely to be overcrowded.

In the past, critics warned the class size law would funnel funds to schools that already have more resources, although officials have promised to prioritize funding for schools with high needs.

But educators and parents have said smaller classes are already making a difference.

“If there's anything magical in education, I think this is as close to magic as you can get,” said Viki Manolas, a literacy teacher at P.S./I.S. 187 in Washington Heights. “Lowering class size and giving students the ability to be seen and heard is a necessity.”

There will be hearings on the proposals over the summer. School officials cautioned that revisions are possible.

Last week, the state Legislature granted the city an extension on implementation of the class size law, with 70% of classrooms required to be compliant by next fall, with full compliance required by the 2029-30 school year.

This story has been updated with additional reporting.