So little sympathy for you it's difficult to express. From some of the more sincere responses I actually thought you'd done something terrible, and from the thread title I thought "oh shit, this guy needs help". How wrong I was.
Don't have secret parties where you invite a bunch of arseholes over and get wasted underage. Instead spend the time growing the fuck up. Don't offer to hold onto weed for someone unless it's your house, even then, think about who it is and why they're even asking. I can't even eloquently express how stupid all this is. (Ideally, don't hold onto weed.)
I can understand the banal irrationality of parents / step-parents, sometimes, but their response here seems fairly reasonable for how premature the news is, and how they discovered it. If you'd just fucking told them. The crass idiocy here is astounding. Hiding beer cans down the side of your bed. I DONT EVEN HAVE WORDS.
Here's what you should have done post-party. Told your parents, and refused to hold onto the weed (or thrown it out and payed him back, if you were too drunk to refuse), inform your "friends" they're all reprehensible twats and should have put some effort into helping you clean up rather than crawling home to feign sobriety. The guy who threw up should have cleaned up his own mess; you should have enforced that.
However having said that, you should have had the fucking forethought to not have the party in the first place.
Stay where you are, attempt to mend bridges, work on paying your mum and step-dad back (whatever this means), get better friends, work on that aforementioned growing up. If you think I'm preaching abstinence, utter sensibility and conservatism to you; I'm not.
Now why I'm so pissed off: because this problem is trivial. So you let your mum and step dad down? Everyone lets their loved ones down at some point. You feel bad, they get disappointed, but if this is seriously the worst thing you've ever done, and you generally get along, you'll get past it.