Multiple issues there.
1. Does not appear to have been a "morality clause" in his contract.
2. He was tenured (see #1)
3. Staff refused to testify against him for fear of retaliation.
4. Students who were spoon fed and graduated without learning the lessons didn't want to testify either.
It's a rock and a hard place, Catch-22 thing. Society demands more and more from our educators, but voters refuse to vote for salaries that would attract the best qualified, or funds to reduce class sizes, maintain and build new schools, etc.
Then there are parents who fail to instill manners and discipline in their children but then expect the schools to do just that - but don't dare take a paddle to the kid or risk jailtime - or assault from the kid who goes unpunished.
In this particular case, I don't understand why the feds or local police didn't arrest him for fraud. Typically, those graduation numbers are fudged to increase funding - funding of taxpayers money.