to be honest I'd be very doubtful your school would take much action against this guy, especially given this is their head boy, they also wont want to look bad, I mean what you described is very vague as well, there is a difference in how people will view 'he grabbed your ass and said get your tits out' to 'he shoved his hands in your pants and told you he was going to rape you', I'm not saying the first one is okay, but I am saying it is a lot more common and will be taken less seriously, if it was the second then you would be more able to pursue a police investigation
I would say to do whatever you want to, I don't think there's a right and wrong, if you're really struggling to deal with this (and trust me I know it's hard to watch someone who made you feel so disgusting and humiliated become some sort of 'golden boy' while you're left to struggle) then I certainly think you should at least talk to someone about it, if you feel like you need to be honest and you need him to be punished in some way to move forwards then do so, but be aware that there is no garantee of action given how long it has been (although you're in a good position given your friends are on side and witnessed it)
I really really really think schools should be covering things like consent, respect, assertiveness etc in PSHE lessons, this sort of thing affects so many people and you hear over and over that girls just didn't know what to do or how to react, I'm not saying its the girls fault and she should have to react perfectly, but the number of these situations that happen really show that young girls need to be given some sort of tool