[Decolliber]

You just have to accept that any car driven by a teenager is going to get dents. Just buy them an old car that does not matter much. When my kids were in high school they drove an old Escort and an old Toyota pickup, both of which were dented to begin with and got lots more dents. And remember that if they park in high school parking lots they will get dents that are not even their own fault. Furthermore there is a lot of petty vandalism in high school parking lots.
The answer is not sanctions; it is making sure the car they drive is not worth worrying about!