When lawmakers forced New York City last year to reduce public school class sizes, many parents celebrated a long-awaited victory. But now, the popular move is running into a surprising opponent: other parents.

At New York’s high schools, classrooms would shrink to 25 students over the next several years, down from 34, coming close to class sizes in some suburban districts.

But what would typically be a major selling point for a school system has transformed into an emerging battle. A growing number of families who want their children to attend the city’s most selective institutions, including its coveted crown jewels like Stuyvesant and Bronx Science, worry their odds could decrease at popular schools with packed classes and little extra space.

The anxieties reflect perennial fights over elite school admissions, one of the most fraught issues in New York’s school system, the nation’s largest.

Many politically engaged parents in sections of Manhattan, brownstone Brooklyn and Queens have fought hard against proposals that could upend the status quo at hypercompetitive schools. This month, for example, a wealthy Manhattan district reversed a desegregation effort that ended selective middle school admissions, reinstating four screened programs after intense parent backlash.

The tensions could mean a turbulent road ahead for Mayor Eric Adams’s administration as officials confront the new class size mandate and a shrinking education budget. Scores of other families and educators, as well as the influential teachers’ union, support the move. They would undoubtedly push back — loudly, and in great numbers — if there were any major rollbacks or delays.

The debates in New York come as more districts across the nation have recently enacted caps on class size. In Los Angeles, Minneapolis and Columbus, Ohio, the issue helped to fuel teacher strikes. Smaller classes can help students, and disadvantaged children may stand to benefit most, research shows, though the extent of gains — and whether there are more cost-effective ways to improve education — remains hotly debated.

Under the New York law, Senate Bill S9460, smaller classes must be phased in through 2028 for all grades. But some families fear the shift could make the often intense school application process even more cutthroat.

High school students can apply to schools citywide, ranking up to a dozen programs. Competition for the most elite schools is steep. Last year, there were 26 applicants per seat at Stuyvesant, for example, and 24 per seat at Townsend Harris, a selective school in Queens. Smaller class sizes could reduce how many students can enroll.

Shane Harrison is the parent of a seventh grade student and says she typically supports smaller class sizes. But at a recent public meeting, where several parents said they were concerned about the mandate, Ms. Harrison said she worried that “for high schools in particular, cutting seats would have a devastating, cascading effect.”

If large numbers of teenagers end up in schools they do not want to attend, “that would be a colossal failure,” she said.

In New York, the vast majority of students attend nonselective schools. But the selective institutions remain, in some people’s minds, an important draw to wealthier families. Concerns that more of them might abandon district schools have at times shaped local policies. Earlier in the coronavirus pandemic, high school admissions criteria were loosened, and fewer students received an offer to one of their top choices. An uproar led to a reversal months later.

State Senator John Liu, a fierce defender of selective admissions who also sponsored the class size bill, said he empathizes with parents’ anxieties.

“There’s going to be a lot of concern, and I understand that,” said Mr. Liu, who chairs the New York City education committee. He added, however, that while the transition may be tough, “that cannot block real progress that nobody will regret years down the road.”

Some schools may receive exemptions, if they have limited space, for example. But the teachers’ and principals’ unions must approve plans, and it is unclear how waivers will work in practice.

One group of families has proposed a compromise: a two-tiered system that allows for lower class sizes at most schools, but exempts others where metrics like test scores are particularly high. Still, many parents believe reducing enrollment at desirable schools could spread out children, help bring resources to less popular schools and promote desegregation.

Beyond admissions, officials will face other challenges, including hiring about 9,000 new educators. School officials have often warned that the law could worsen the system’s financial constraints and require tough trade-offs, and Mayor Adams has called it “really unfair.”

“Brooklyn Tech, Stuyvesant, Bronx Science — they’re not having a problem with academics,” Mr. Adams said at recent town hall. “They’re not having problems with enrollment.”

But many families and teachers say they hope state legislators stand firm and give them a reprieve from crowded classrooms in places like Fresh Meadows and Bayside.

“This really weighs into the decision of how long I’m going to teach,” said Jake Jacobs, who works at a Bronx middle school, adding that educators “would be able to give students more individualized attention — whether it’s a high-performing school or not.”