All you need is a good Alibi. Get a friend or cousin to say you were hanging out with him at the time. This is some big misunderstanding. This is a case of underage drinking... not murder. I'm sure your town/county whatever doesn't have the resources to run an intensive investigation to prove an offense as small as that occurred. Ok, so if someone else mentions you were at the party... you deny it. You could always claim that they were just setting you up to take the fall with them. That is unless you know some dumb bitches took pictures of you there. (Chicks with cameras suck). Just lay low for a while. And see how it pans out. Don't talk about it, unless someone really mentions something directly to you.
Dude you're talking about something so small here though. The most they can convict you of is Possession. That's NOTHING. Maybe a $50 fine and 10 hours of community service. Colleges don't look at the shit or anything when you apply. And your car insurance company won't really find out either. Seriously, a MIP is in the same league as a speeding ticket.
I'm actually 21 now. But when I was in highschool, this dumbass that I graduated with was the biggest fuck up ever. He has 2 underage drinking violations, a weed possesion, and an underage DUI. I mean he was kinda fucked with fines and community service, and his license was revoked for 30 days. But honestly, nothing terrible happened to him. His parents fucked him over harder than the law did. They actually didn't let him go to college the year after he graduated.