Mitchell's pretty much got this topic won. So you both like each other, that's cool. Good for you. The question is, are you both willing to wait for each other and remain faithful to each other until you can be together?
I was in the same boat as you a couple years ago... we swore we'd wait for each other. We thought our love was stronger than the hundreds of miles that separated us. But I was wrong. Over time, the quality of her life at home seriously deteriorated, and her patience finally ran out. Things changed. It's a real shame, too. I was more than willing to be with her forever.
What you gotta ask yourself is is it worth it. You're only in middle school. Do you really want to forsake all your possible relationships not only in middle school, but in high school as well, for this one girl who doesn't live anywhere near you? Does she mean that much to you? Are you willing to tough it out and wait for a chance where you can be together in real life? That may take several years to happen. It might not happen at all.
If you want my honest opinion, I would tell you to break it off. Don't say you'll date her (and be honest, you can't exactly take her to the dance if she lives across the country), but stay good friends with her. You never know what could happen down the line. You might end up meeting someone who can give you the things that this long-distance girl can't.
It can get tough to stay faithful to a girl you never see. Talk to her about it and get her opinion. She just might be thinking the same thing you are.