Hey man,
This has happened to me many times, to a point where I always abstain from doing anything with a girl from my social circle. That being said, it can still happen and here is how to deal:
Firstly, act like it was the most natural thing ever and not something worth talking about. You wanting to go up to her after class to explain the “situation” is a great way to make her feel dumb and hate you forever

.
Keep going on with life as before. If she mentions it, just smile and brush it off as something that can happen sometimes between
friends . Girls are not dumb, they will understand if you see it as a fling (especially with alcohol in the mix), no need to explain! Just be cool.
Second, i’m sorry but you shouldn’t hook up with another girl in front of her until you see her with another guy first. This is to make sure she has no more unfounded hopes of being with you. Getting with someone else isn’t classy and will probably anger her, if only for that reason.
There you have it: the formula to keep her as a friend. The “no hookup” rule sucks, but hey, find girls outside your social circle!
White Panda
Tinder Advisor blog