What comes around, goes around.
He will get his in the end. It would be really nice though to witness it, now wouldn't it!
If you are not of legal age to ride your motorbike to school, then don't ride it. That's what weekends and "private property" are for.
If you do not care if your motorbike gets impounded then ride it anyways. (Hey, it's not my $$$ you would be loosing....)
As for your engine getting vandalised during school hours, it is very, very likely to happen.
Do you want to risk getting it destroyed by some jealous, idiot, punk?
Think it over.