Simple: Confront her. Firmly. Don't let her walk away or treat you like ****. Ask her what is wrong and tell her that if she is going to continue to do this then she can consider the relationship over. Then listen to what she has to say.

if a girl ignores you that means: you did something wrong (but you dont know what ofcourse becouse girls can be mad very fast x3 yes i know so, im a girl XD) or she just dont wanna move on with you and doesnt dare's to say it to you.

maybe you should just say her right away: you still wanna be with me or not? so i can move on too

The solution to this problem is rather simple. It doesn't require you to be a **** in return like most of the suggestions. When you have a moment that it's possible to talk just say "Hey, I can tell something is wrong. Let's talk about it. We need to work this out and get over it together, or we need to work it out and get over it separately. If you want to end this, you need to tell me."

If she still won't talk, end the relationship. Relationships are built on communication. If she won't communicate, she's not interested in the relationship.

She prolly met someone better and is waiting for you to go away. Find someone else.

Girls/guys just need to learn not to hurt those they care about,

Won't know whatcha got till it's gone.

TL;DR
She is fcking another guy

If you're still "seeing her in school," then don't worry about it. There are bigger things in life.

For sure, it may hurt now, but in the long run you are better off without her. Trust me, especially since you are still in high school (assuming since you have a car), that there are much better things to come. I know it's gonna suck, but try not to think about her or what she's doing.

Involve yourself with something else, maybe even someone else. Take what you have learned from this relationship and apply it to other ones so these kinds of confusing situations aren't as confusing. You'll be fine, I know it, and if you're going to college soon, you'll be more than fine.