I can tolatly understand what you are going trough..all the hopes yoou had just senk? Hmm. Let me answer your question now.
Well, I thihnk that I dont have all the information to have a clear opinion. It might be that her freind was really in need, and that she chose to be by her, or it really might be that she blew you of. I guess your mind is set on the second version, which naturally makes you feel angry.
When you calm down a bit and if oyu are really up to this girl, I would sugest to ask her out again. Or if you do not want to talk to ger, you can just try to observe carefully for a week or two how does this girl reacts whan you meet in school, maybe try to sit by her at lunch, and see if she avoids your eye contact, does she avoid to say hallo, or is she still very friendly with you. People who lie usually have guilt feeling afterwards, or if they are not comfortable with our presence they may say kind words but you can notice thir feelings and thoughs in their body language.If you decide to ask her out again, and If you get refused, or if she again finds an other reason and do now suggest an oter day or time, well then sorry dear, I guess it is better to go on. I know you will not be happy then, but al leest you will know what happened whan she changed her mind.
I hope this helped yoou....