Here's my take on this. I work in property management so I hear parking stories all of the time.
It sounds like the faculty member didn't like the car in "their" spot and instead of calling a tow truck which would cost you $$ they decided to be passive aggresive and move the car to give the illusion that it was stolen and hopefully your son would learn not to park there. In the real world, the van would have been towed.
Just this morning I had to put one of those nasty stickers on a car parked in my co-workers reserved space. The person who parked there came up screaming that we vandalized their car. I pointed out that we had a right to cite and could tow the car under the law and also pointed out that there are signs all over the property absolving us of responsibility for any cars or property in our lot. If the school was smart, the school will have plenty of signs in the parking lot regarding the parking rules or at least a list in the student handbook.
To the people who say get a lawyer: Are you kidding me? Moving a car did not cause any emotional, monetary, or physical injury. I would be very weary of any lawyer who would actually take on a case like this. The taxpayers' money was already wasted on the police filling out paperwork and now you want to tie up a courthouse, city attorneys and other resources?