The movie Won’t Back Down has already accomplished the impossible: Making teachers’ unions demand strict accuracy in films about classroom education. For decades, Hollywood has been making movies that show teachers as superhuman caring machines without a peep from the unions. That math teacher played by Edward James Olmos in Stand And Deliver, the one who took over a classroom of kids who couldn’t do simple arithmetic and in nine months had them aceing calculus exams? History does not record a single union official complaining that, in real life, that process took several years.

Won’t Back Down, however, is a different matter. It stars Maggie Gyllenhaal as a single working-class mom driven frantic by the lack of help her dyslexic daughter is getting at school. Frustrated at every turn — administrators won’t let the girl transfer to another class, and the school superintendant won’t even see her — the mother is startled to discover that her state has what amounts to a nuclear option: a law that allows a majority of parents to take over a failing school. Enlisting the help of a sympathetic teacher (Viola Davis, who just won an Oscar for her role in The Help), Gyllenhaal launches a campaign to do just that.

Won’t Back Down is tough on teachers’ unions, treating them as an obstacle to school reform. And in return, the unions have labeled the movie a big fat lie that, for instance, doesn’t once show kids taking tests or parents at PTA meetings. Worst of all is its claim to be “inspired by true events.”

“That conveys the message that parents and teachers took over and ran a school somewhere,” wrote Rita Solnet, a founding member of the teacher-union front group Parents Across America, in a widely reprinted blog item. “That never happened. I suppose that sells better than opening the film with, ‘This is Fictitious.’ ”

Though laws similar to the one in the film are real — known as “parent-trigger laws,” they exist in seven states and are under consideration in several others — Solnet is correct: Parents have never managed to use one to take over a floundering school. That’s because when they’ve tried, teachers’ unions and school administrations, which have a common interest in protecting their failures from mere parents, have staged dirty, underhanded campaigns to prevent it.

Won’t Back Down was indeed inspired by two cases in California. One is McKinley Elementary School in Compton, which for six years has been on the state’s listing of failing schools, languishing in the bottom 10 percent — less than half of the students graduate, and only one in 50 goes on to college. In 2010, tired of beating their heads against a brick wall of administrative indifference, the school’s parents spent four months gathering enough signatures to activate the trigger law.

The teachers’ union struck back like a wounded snake, sending operatives out to get parents to remove their names. Those who had signed were told they had to show up during school hours with photo IDs so their signatures could be verified. (The progressive canon of faith that requiring photo IDs for voting is repressive and racist was apparently under temporary suspension.)