We've taught our children right and wrong through consequences. When we were in the borg the elder body didn't like that style of parental training but I don't give a s**t about their thoughts.
If I get stopped for speeding I will acknowledge driving too fast. Thank the officer for stopping me. If I had continued driving fast I could have had an accident injuring myself or others. Accept the ticket if given and learn the consequence from the experience.
Explain this to your son and then go to the school. You can calmly explain that this incident is an opportunity to teach responsibility on how to handle matters. Your son can calmly accept the ticket. Ask what the school policy is for wrongfully parked vehicles. My guess is that the policy states that the vehicle is to be ticketed and/or towed. Since the car was illegally entered and moved, the school policy was not followed. AFter this you can then ask and expect that the person responsible for moving the vehicle will be held responsible for not handling the matter according to school policy. This can be brought up to the school board.
However if the person comes and gives an appology for entering and moving the vehicle you can then accept it after being assured that this will never happen again.
Consequence for both sides.
Anything beyond this will just cause difficulties until your son graduates. After March of 2005 the MPD hasn't been the same. You probably won't get much help from them in this case.