Al Gore's film-maker moves from "An Inconvenient Truth" to an even more inconvenient truth about failed unionized schools

It’s the film the teachers unions don’t want you to see.

The revelatory documentary “Waiting for ‘Superman’ ” opened Friday to parents’ cheers — and union howls.

The film follows five families trying desperately to escape failing traditional public schools in favor of charter schools — and it profiles education reformers rebuilding a national school system that’s in ruins.

The unions panned the flick, naturally: It exposes how they drag kids down into the swamp, spotlighting how bad teachers are passed from school to school and how all-but-automatic tenure allows even the worst teachers to stay on the job.

But the most crushing scenes show the public lotteries that determine which kids will get the painfully few spots in charter schools they’re aching to enter. Tearful audience members called it “heartbreaking.”

Unions called it “manipulative [and] deceptive,” and bashed charters for trying to “lure” kids with the “false and absurd” premise that school choice matters.

False and absurd? Tell that to the thousands of parents who line up for the lotteries every year. And tell that to the kids stuck in failing schools.

Some educators are already standing up for the film. As one Michigan teacher and union rep told The Post: “Some of the union stuff is a huge obstacle to progress.”

To put it mildly.

What’s even more encouraging is that the film was was made by Davis Guggenheim — the fellow who was behind Al Gore’s “An Inconvenient Truth.”

That is to say, a committed Hollywood liberal. And when those folks reject the union label, it means that the bankruptcy of teacher-union-driven public education has become glaringly obvious to even the willfully blind.

Upon such revelations are real reform movements built.