They either say what the decision says they say... or they don't.
It would be a simple lawsuit for him to win if he actually has a stack of parental permission slips for all of the student work used in his book and the permission clearly includes a self-published and copyrighted work. It would be an open and shut case if they put into evidence documents that so clearly contradict their finding of fact... an easy wrongful termination suit for the union to win for him.
When does he indicate that such a claim will be brought to court?
On edit - Another of his blog posts admits that he doesn't have permission slips for all of them and that they were "poorly written". He believes that they must have fallen out of the folder at some point.
If you're going to publish minors' work in your own book, you need explicit parental consent for that (not just for publication) and you need explicit administrative permission (not "I mentioned it more than once and he didn't say no"
.
Administrators lie. Like a rug.
Some do... and so do some teachers. In this case I don't have anything to go on except what I read in his blog... and he's the only one that I've seen lie so far.