Class size doesn’t matter.

That’s a common theme among the so-called reformers who contend that an effective teacher can teach a class of any size, as if there is no difference in opportunities for teaching and learning in a class of 30 or 35 compared to one of 20 or 25.

This goes against both research and common sense, a point reinforced by a recent research review, “Does Class Size Matter?” published by the National Education Policy Center. It found that class size “is an important determinant of student outcomes,” particularly for low-income and minority children.

Increasing class size harms not only children’s test scores in the short run but also their overall development and success in the long run, the review found.

Class-size concerns in our city come to the forefront every fall as teachers and students return to find some classrooms so crammed that students have nowhere to sit. In many other cases, the situation is less dramatic but more insidious. As the Mayor’s Management Report for 2014 shows, average class sizes are continuing to steadily creep up.

On Sept. 18, the UFT used the expedited grievance procedure in its contract to file more than 3,500 grievances over class sizes in excess of contractual limits. But the fact that such violations now occur like clockwork each fall shows that this is a problem that requires more than a contractual solution.

Bringing class sizes to the reasonable level that students deserve requires a broad recognition among policymakers of the need to make this a priority and a strong commitment to make that happen.

It happened with prekindergarten.

As with class size, studies on the value of high-quality early education have been piling up for years, but no one thought it was possible to move the needle on such a complicated and costly initiative.

Then we elected a mayor who made prekindergarten a top priority, and now full-day pre-K is a reality for most 4-year-olds.

Our city can do the same with reducing class size. We only need a plan to raise the funds and find the space — and elected officials to champion it.