New York City’s teachers union called for raising taxes on 90,000 absentee luxury apartment owners to pay for smaller class sizes for kindergarteners through third grade.

Michael Mulgrew, president of the 200,000-member United Federation of Teachers, said today that tax breaks on condominiums and cooperative apartments should be rescinded for non-resident owners, which would eventually provide about $900 million a year to pay for the more intensive early schooling. The city currently spends almost $25 billion a year on its schools.

Mayor Bill de Blasio, 53, who has called income inequality a defining issue, said he’s considering s similar proposal that would raise taxes on non-resident-owned apartments valued at more than $5 million. The mayor lost a fight over raising taxes on the wealthy to pay for universal all-day kindergarten. Instead, the program was funded by the state.

Mulgrew said his plan would start with 100 schools at a cost of $30 million and expand over several years to cost $900 million. The students would sit in classes no larger than 15 students, he said.

“Out-of-state and foreign millionaires and billionaires have bought New York City apartments as investments rather than homes, yet they get to take advantage of excessive real estate tax breaks,” Mulgrew said in a statement. He cited research showing young students improved their learning skills when in classes of 15 or less, about half the size of many elementary school classes now.

The property industry’s lobbying arm, the Real Estate Board of New York, says raising taxes on pied-a-terres will scare off investors who fuel a business supporting more than 500,000 jobs and generating 40 percent of the five boroughs’ revenue.