it could be the things you experienced triggered your anxiety, it could be that it would have happened anyway... it's not really useful to find 'blame' as it will stop you moving on, you feel what you feel and if what you feel is hurt or ashamed or angry or whatever, it is useful to try and deal with that and talk to someone about it so you can start to move on, but blame is not something to move on from, it ties you down with negative feelings
unfortunately your head of year probably didn't have many options to take, I doubt he ignored it on purpose and as other people have said it isn't an uncommon thing to happen, 99% of girls have probably had their ass grabbed in a club, that doesn't make it okay, but it does mean that he wouldn't have taken major action - particularly if the others either denied it or said it was a laugh and not serious
in response to your actual question don't contact your head of year, he probably wont remember, he probably didn't ignore their behaviour maliciously, and he probably has no idea they upset you so much... plus he may not even talk to them anymore... it would be very childish and quite selfish to try and make him feel bad for something that you don't know the whole story of