Regardless if what form the drug was in, they should have arrested him, not have an employee drive him home since he was intoxicated. They should have hit him with multiple charges such as child endangerment, possession of the THC, and possession of paraphernalia at the very least. I'd probably throw in contributing to the delinquency of minor also since it was at school. If he is too intoxicated to drive himself home, then he is too intoxicated to be in charge of children.
Again, I'm sure if it were a student, they'd be arrested. They probably would have arrested any other adult too and thrown the book at them while making it VERY public that this type of behavior won't be tolerated.
This entire situation is embarrassing to say the least.