You are doing your son a disservice by letting him get away with bad behavior. What you are doing, by not acting, is giving him permission to misbehave again. So your problems with him will multiply. And don't use ADD as an excuse, if in fact he has ADD. Most of the diagnoses are erroneous.
You need to make the punishment fit the crime.
If he did indeed scratch the car (ask for proof) then have him do chores around the house or school to "earn" money to help pay for the repairs. Even if he doesn't make any money or give it to anyone,this will still teach him that he can't be bad without consequences.
Never give a punishment that doesn't fit the crime, and never give a punishment without first making sure that he did it.
I have ADD and the worst part for me was constantly being punished for things that I did not do, or being beaten for those crimes that I did...
I was once given the strap in grade 2 because I had gotten bored and started reading ahead of the other kids. We used to take turns reading aloud. Anyway, it was my turn to read and I had no idea where they left off. I was at the end of the reader. I got the strap and my mom was called into the school to deal with me. I got the belt at home. Does that seem fair? No.
So your son should not be strapped,beaten,grounded,or have any other punishment that does not fit the crime.
Make him apologize and wash the teacher's car or something...but only if there is proof! He will resent everyone if punished for something he did not do. Perhaps the "friends" did it and blamed him? Perhaps that he scratched a different car and his friends scratched the one he is being blamed for?
Take him to the parking lot and do a re-enactment. Tell the principal and teachers what you are doing.
Perhaps you should leave it up to the school to discipline him...after all,it did happen there and not at home.
I don't believe in double punishments...it's not fair.
And if you all discipline him you will lose his trust for fair play.
Good luck and let us know how it goes.