Oooh! I don't think I could have a brass enough neck to appeal that.
Stupidity in climbing the scaffolding.
Possible damage done by climbing scaffolding.
Danger posed to himself and others, increased if he needed rescuing.
Add to that being pissed and presumably belligerent when brooked.
There are all sorts of reasons that would be a flat "sod off" from the school.
And as many if not more from you. You need to parent this one. He has to accept the consequences not his actions and you need to steer him to accept that the whole fault is his, that the school are absolutely correct with their response and that he has to do some serious apologising before he sets about finding a way to correct any damage he has done to his sitting of exams etc.
I suspect the school could be persuaded to allow him to sit exams there, heavily chaperoned, which cost them even more money and inconvenience. But he has to brooch that with them, fully understanding that they might say no!
Basically here is where he has to step out of his carefree childhood and get to grips with being an adult. Your task is to ensure that he makes that step this time, not after the next fuck up, or the one after that, or after that etc.