Some months ago we made out on a school trip and because I was a little distant she thought I did not want a serious relationship and when I asked her where are we going from here se said nowhere 2 weeks ago she hit on me but I didn't got it in time and she thought that I really don't like her and moved on with a 24 year old guy (she is 17) and she says she can't think anyone but him but we talk a lot and when I am in class and say "well I acted like a jerk back then but when you break up with your boyfriend and be with me everything will be perfect'' she says ''yes'' blushing.She knows I like her and I told her and I think she kind of likes me too but says she only thinks of him.What should I do,I am very nice with her we talk all the time in class... she has some minor problems with her boyfriend because he is older than her and she doesn't want sex now so how long do you think is it going to last and what should I do?

They may stay together forever and be together when they are 80, no one knows, most relationships have problems, and if they work them out, they last.

The fact she is talking to you, may mean she is considering leaving him. So tell her how you feel and see what happens.

I have already told her and she knows,but she says that she only thinks of him

You just remain true. Secondly, it's a good thing if you tell her your feelings. If she doesn't respond, have patience and wait for sometime. If even then she doesn't show any inclination towards you, maintain a bit distance from her and do not talk to her all the time(When you lose something you relize it's importance and she may realize yours... ). Then, if she misses you and calls on you, she wants you. If not you should move on...

Here are the guidelines:

https://www.askmehelpdesk.com/dating/guidlines-what-do-do-if-person-like-already-relationship-463250.html

IF she already knows, then back off, let her make the next move. As long as she knows how you feel, the ball is on her side of the court.