In July, New York City Mayor Bill de Blasio made a deal with state legislators: The mayor would retain control of city schools for two more years in exchange for a commitment to increase the number of charter schools.

It’s no secret to New Yorkers that de Blasio opposes charter schools—public schools that typically operate independent of traditional school districts—and he has engaged in high-profile efforts to slow charter school growth.

But new research suggests that what The New York Times calls his “unwritten concession” on charter schools may work out well for taxpayers and students.