Lowering class sizes in New York City is not an experiment, a wish list item, an “unfunded mandate” or just another expenditure competing for city Department of Education funding. It’s the law.

To be sure, giving New York City children smaller class sizes — which their peers across the state already enjoy — is not an inexpensive or easy undertaking. It requires additional teachers, classroom space (including new construction) and intensive planning and monitoring. The new class sizes, which the DOE must implement in 20% increments over five years, are 20 in kindergarten through grade 3, 23 in grades 4–8 and 25 in high school.

Albany has done its part by at long last fulfilling the promise to fully fund Foundation Aid. New York City now has the funds to provide smaller class sizes for optimum learning.

The mayor and the DOE must stop treating the law as if it were optional. In this first year of the five-year phase-in, they must redouble their efforts to implement it for the benefit of the city’s children, educators and communities.

About 39% of classes are already within range of the new limits, but what about the later years? One thing is for sure: With space at a premium, the city must stop co-locating charter schools in public school buildings.

We do not dispute there are other needs in city schools as well. But smaller class sizes have been a top priority for New York City parents and educators for decades. A new universal standard for class size in all New York City public schools will yield dividends for our city’s children for generations to come.