Bob Bowdon knew that tackling the damaged New Jersey public school system in documentary format would give him plenty of raw material. He just didn’t know precisely how much material he’d have from which to choose. “It became kind of like a drug, you’re finding more and more and more of it,” says Bowdon, the mind behind The Cartel, an excoriation of the teachers’ unions ruling New Jersey’s students. “Who at the outset imagines you’d be finding janitors making six figures, or a superintendent making $470,00 one year, the same year he was fired?” Or, for that matter, a veteran...