Two years ago, New York State lawmakers passed a measure that required New York City’s public schools to lower their class sizes at a time when the city was in the midst of profound change.

Its student population had dropped dramatically during the coronavirus pandemic, but the arrival of thousands of migrant children had leveled the population loss, while bringing new challenges.

As the city struggled to be an affordable, attractive place to raise children, leaders in the school system were now tasked with figuring out how to reallocate smaller numbers of students among classrooms — an expensive directive that often requires more space and staff members.

But slowly, some class sizes dropped under the required limits. Teachers rejoiced.

“I have more students in my class that have very limited English, and I feel like this has been a huge help for me, to help them and for them to be more comfortable in the environment,” said Suzanne Aruti, who teaches second grade at P.S. 135 in Queens. She said she has 20 students, down from the 32 she used to teach.

For years, city officials and teachers sat on opposite sides of the debate over lowering class sizes. Teachers wanted the ability to give students more one-on-one attention, while the city felt that there were more cost-effective ways to improve education.

Now, two years into the effort to comply with the class-size law, the Department of Education has adopted a new approach: For the first time, schools were asked to create their own plans to lower class sizes and apply for funding to help them do so. Before, the D.O.E. used a formula designed by the State Legislature to send funding to schools identified as places where class sizes could be reduced.