It is entirely your decision whether or not you choose to tell your gf about what happened. Either way though, just make sure you are doing it for the right reasons.
If you choose to tell her, dont do it because you feel guilty or because someone else has told you they think you should, do it because you want to be honest, you want to talk things over, you want her to know how much of a mistake you think it is.
If you decide not to tell her, then make sure its not because you want to hide it from or or that youre scared of how she will react, do it because you dont want to hurt her uneccessarily, because you want to put the past behind you and really give the relationship a go, you know its a mistake and it wont happen again.
Before you do anything though, I suggest you think hard about what it is exactly that you
are wanting, whether it is a relationship with your current girlfriend, whether you want to maybe have a break from dating and be single for a while or whether this girl you kissed is worth trying to have a relationship with.
Only you know how much each person means to you so only you can decide what to do.
I hope things turn out alright, and goodluck for your exams!