Hey Reddit, what's the best way to get back at an ex girlfriend?

background information:

I'm in high school and we both "hang out" with the same group of friends, our group are fairly close and we do a lot of stuff together.

Her and I became attracted to each other after a while, so we decided to go out. We went out for eight weeks before she dumped me, "it wasn't working" apparently.

The next day I get texts from her saying it was a big mistake and she wanted to get back together with me. Me being the idiot I was, I decided to give it another go because i had grown quite fond of her.

Two weeks later she breaks up with me again, this time it was "it's for the best".

Our group are close so i wont be able to just "avoid" her. I now hate her so i want to get back at her the best way possible, any ideas?

Edit: Yeah, you guys are right. I guess I should be the mature one.