How can i get back at her?

As much you want someone to come up with an evil plan to teach her a lesson. You're not going to find it. If she's a liar, she'll end up getting caught at something else. The best thing to do is to be as happy as you can be EVERY time you see her, you might even say hello. As a teenager it's a hard thing to do, but usually when you try getting back at someone, it ends up being thrown back in your face, I'm speaking from experience.