Teachers and parents know it, and research has proved it: Significantly smaller class sizes, particularly in the early grades, can lead to strong gains in achievement. The benefit is greatest for poor and minority children.

In New York City, where 78.9 percent of students are poor and 68.5 percent are African American or Hispanic, it only makes sense to implement this proven strategy of reducing class size in kindergarten through grade 3.

It especially makes sense now to ensure that our schools can build on the gains made by our youngest students in prekindergarten.

That’s why the UFT is proposing that the state take action. Closing tax loopholes for absentee landlords of luxury condos and co-ops in New York City would raise the money to pay for the smaller classes we need.

Corporate education reformers will say that class size doesn’t matter. They will pull out the tired argument that student achievement depends entirely on the teacher and that nothing else — not poverty, not the number of students in the class — affects student learning. Remember Bloomberg’s assertion in 2011 that city schools would improve if he could fire half the teachers and double class size?

Research shows otherwise. In the most famous class-size study, in Tennessee, students who were in smaller classes of 13 to 17 from kindergarten through 3rd grade gained on average 7.1 months in additional learning, nearly a full school year. The gains were also significant for students who were in small classes for some but not all of those early grades.

New York State in 2007 settled the Campaign for Fiscal Equity lawsuit with a plan to fund class-size reduction. But the Bloomberg administration didn’t use the funds to target class size and the additional funding was pulled back in 2009 with the financial crisis.

Acting now to end tax breaks for absentee owners of luxury condos and co-ops would produce a fairer tax system. Using the money for smaller classes would produce better schools.