When he says a bit of shenanigans took place - was he there when they took place or does he just know that they took place and who was responsible but wasn't actually there to witness them (in which case he knows by hearsay rather than directly?)
I think it makes a difference - if it is the former then could he say that when he left the loos the damage hadn't been done and therefore the school are being unreasonable to expect him to be able to say he knows exactly who did it as he wasn't there to witness the damage being done in person. And that by checking the tape as to when he left the toilets they could narrow down the group of possible suspects.
Not so easy to do that if he was actually in there when the damage was being done (unless he was using another cubicle and could therefore say he wasn't able to see who was doing the damage and as there was lots of shouting (or none) he couldn't work out who was doing what.
I can understand the school are hacked off about the vandalism and want to get money back as well as punish the perpetrators. However, given that it is so close to A level exam time and given that there is a very good chance that some or all but one of the ten suspects are innocent, then it is also worrying that they are willing to stress out up to nine innocent kids which could have a big knock on effect on their exams and thus a big knock on effect on their university choice or other post school plans, and thus their lives and careers beyond that, if my child were one of those that were innocent but being punished in this way, I would be spitting furious that the school are effectively hoping that they could guilt the guilty person into admitting their wrongdoing at the expense of everybody there - given that as he already hasn't, there's a chance (making wild assumptions!) that as he was happy to damage the toilet doors then he might not be bothered about others being punished at the same time he is if it means a lesser punishment for him. (I assume that if the school find out for definite who did the damage then they will be punishing them more strongly? Maybe not.)
If your ds is a good kid who has never been in trouble in school before and he wasn't involved then it is particularly harsh to punish him like this. I always thought that you were innocent until proven guilty, not innocent until you got down to the last 10 suspects and then punished them all when you couldn't figure out who the perpetrator was. It's a horrible thing to happen to him at an already stressful time and not something you'd want on your school record.
I'd also put it to the school that what are they expecting your son to say if he doesn't know who did this (by witnessing directly) - are they expecting him to guess? Because if he did and guessed wrongly then that has huge repercussions for your ds, the person accused and the person that gets away with it. They are putting him (and others in the same situation) in an impossible situation as they can't dob somebody in if they don't know. Again this doesn't work so well if he did see what happened but that's not clear from your op. Given that he hasn't said who did it yet, the school are not to know if he saw what happened or now so theoretically it's a valid question.
Sorry that's all a bit muddly but if your ds is worried about repercussions from others then that's a genuine additional stressor that the school haven't mentioned from what you've said that they will deal with. And what happens if somebody else decides to name him rather than the guilty party for example as they decide they are more worried about the guilty party taking revenge on them or think that if several different people are named it will confuse the issue more...