So in hindsight, I think maybe you should've just waited a couple weeks and then asked her if she had gotten her new phone yet. But on the other hand, it was a pretty lame excuse, I mean a few weeks is actually sort of a long time, why couldn't she have given you her number and then given you the new one when she got a new phone? Maybe she was telling the truth, she just wasn't thinking things through all the way and she just blurted out "there's really no point...". I know I've done that before.

I always try to give people the benefit of the doubt. You never know when someone is telling the truth and when they are lying, the best thing you can do is have confidence in yourself and take what they say as the truth until they give you reason to believe otherwise.

It sounds like the girl is interested in you, and even if she's not you have a good opportunity to make friends with her so either way you win. The next time you see her you should definitely ask her out to dinner or something, maybe you can say it's to celebrate the end of the class or something

I hope my lame advice helps and it's not "friend zone" advice this time