Thousands of city teachers will rally Wednesday afternoon in Foley Square to demand that Mayor de Blasio not stand in the way of opening good new public schools in neighborhoods that desperately need them. Why is the mayor resisting? Because the unborn schools would be charters — and de Blasio’s teacher-union allies are desperate to ensure that they never see the light of day. The rally has the same aim as last month’s huge march in Brooklyn by New York parents and kids: to support a rapid doubling of the number of students served by charters to 200,000 — which would be 20 percent of the city’s public-school enrollment. All de Blasio needs to do is OK the new schools’ opening in space that the regular public-school system controls, but doesn’t need. Data from the Independent Citizens Budget Commission show the city has over 150,000 open seats’ worth of space. That’s more than enough for 50 new charters to open — and bring the opportunity for a good education to children whose families now have no good options. Providing that hope is the daily mission of the teachers at the rally: They know charters work, because they work in charters. New York’s charter schools deliver for the city’s minority children — unlike the failure factories in the mayor’s various “school turnaround” programs, such as his Renewal and PROSE schools. In districts where the regular public schools are the city’s worst-performing on state math and English exams, black and Hispanic charter-school students are achieving proficiency. If the city won’t let the new charters use the empty space, then by law it must pay the rent on whatever space they find. But that can delay the schools’ opening by a year or more, as there’s little classroom-ready real estate except in sites the city controls. (And the charters must also raise the funds and take the time to convert the space.) In short, the mayor would be denying opportunity to needy kids and wasting city funds — all to serve his own political needs. What’s progressive about that?