Take my advice with a grain of salt, as it is only my advice.
I am sorry that you're having to go through what you are. It is never fun finding out that you may have been cheated on; and it's not fun, the thoughts that go through your head. Whereas one second you trusted this person completely, the next, you're questioning every action they make. Nothing makes sense, and everything seems like a confirmation of your worst fears.
My advice?
First, take a deep breath. Go somewhere, go to the gym, go get high (you'll never,
ever, hear me make that suggestion under any other circumstances), go read, go pray (worked best for me). Do something to take, however momentarily, your mind off the situation. The more you sit and fester, the worse you will feel.
Second... give yourself a few days. Tell her that you need a day or two by yourself, alone, to focus (on school or something). The more contact you have with her, the worse you will feel. Better yet... take a break from her friends, too. This may seem counterintuitive... but you don't know exactly how trustworthy her friends are. Right now, you aren't thinking too clearly. You're blindly reaching out for someone to trust, for someone to tell you what you want to hear. Unfortunately, this is an opportunity for some real a$$holes to take advantage of your emotions and confusion.
Third... that step is for you to decide. It sounds to me as if you've already made the decision to break up with her. It also sounds like this relationship is hard on you as it is, without the burden of knowing the truth about this situation. If I were you... I would seriously rethink the relationship in totality and ask yourself if it's worth it. Now that you're suspicious, and apparently with some good cause... it will be extremely hard for you to regain your sense of trust; and, for her to regain your trust without knowing everything about the situation.
My biggest amount of advice... take the high road. If she is cheating on you, she doesn't deserve the time of day. May hell fall upon her with all its legions.
However, you have no proof. Until then, you have no right or justification in treating her with any disrespect or malice. While it may make sense, now, that she is cheating; you don't know that for sure. If you decide to break off the relationship, do it in a way that leaves you both, and the relationship, some dignity. While you may be angry at her, don't burn any bridges, especially with her family.
Again, it is a tough decision to make. I wish you good luck and godspeed. Let us know what happens.