EVENTUALLY owned up? That's the bit my child would've been in most trouble with me for. Potentially letting someone else take the blame? No! that's not on.
Plus recruiting others to do it.
So I suspect the reasons are three-fold
A the initial idea
B encouraging others to do it
C NOT immediately owning up when asked.
I don't think it's a harsh punishment, I'd also be interested if he's done similar before and what punishment the others got (although that's actually not your business).
It's yet another case of parent not backing up school when THEIR child has messed up! Instead of focusing on the "unfairness" I'd suggest focusing on teaching your child how to behave appropriately at school and in particular teaching him to have some honesty and integrity.
Honestly if my child had done something like this AND been reluctant to own up she'd have been grounded that weekend.
"I do think its unfair that he's taking the punishment for 20 other kids though." You have absolutely no grounds for thinking this!
"They did it with whiteboard pens." OMG they're murder to wash off!
This is the first incident of bad behaviour like this that involved any other children so already had behaviour problems in the past? That weren't resolved very quickly and you wonder why the teacher wanted to make clear to him this was unacceptable? Sorry but the more you wrote the more I think you need to wise up, support the school and sort his attitude out!
School are ABSOLUTELY right to deal with it sooner rather than later.
They've come down harder because he is not learning NOT to behave badly and I'll wager it's partly because he's very much getting the sense from you (even if you're not verbalising it) that you aren't supporting the school and don't think that his behaviour is "that bad" except it is! And if it's not dealt with now it will get worse - and whatever you think about the school or what other people think of your son, the fact is if YOU don't deal with this YOU will be letting him down. Being taught to value education is hugely beneficial to your son, allowing him to continue to not take it seriously - would be doing him a massive disservice.
I'm not a teacher but have been in roles of authority with children and have dealt with children parents like this. They have to be dealt with a little bit tougher than others or else the behaviour escalates and by the time they're at high school point it's MUCH harder to rectify!
I'm highly sceptical of his "bad behaviour" earlier in the term being as mild as you describe.