Okay, little bro. This is where I fill you in.
Just be up front with her: (now, I warn you. This is either going to work, or going to fail; it's up to you!) Be all: "Okay, girl, let's get something straight: do you wanna do this, or do you want to do this? Why are you trying to do me like you do?"
In brief, you just want to act like you don't care about her. Just be really cool about it; if you don't know how to act cool, just watch the first three James Bond Movies -- Dr. No, From Russia With Love, Goldfinger.
She's only an 8th grader. There will be more girls in your High-School career.
If you think you HAVE to have her, then you're probably not going to have her. Why? I don't know. That's just how these things work out, kid. Tough dookie. If it doesn't work out, best you can do is use it so that you can get with other chicks easier. Go back and read the advice I gave you a while ago (you know which one I'm talking about).