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At recess in the playground or wherever, just approach her, and simply ask,
"(insert name here) Can I just ask you something privatly?"
If she follows just come out with it. E.G.:
"Hey, look, this might sound a little creepy, but....well....I really like you. There's just something about you that I havn't been able to work out yet, but I know I really...really...like you. I was wondering if...maybe...well, we could go somewhere together one day. Just anywhere that we could spend a little time together. So?"
Then wait for reply. You will know what to do from there on.

Trust me this works. I have told many a 13 year old to do this, and it works. If she rejects jsut say:
"Yeah, sure that's fine. But....the offer is always open. *smile* Thanks."
Then pick your sorry ass up off the ground and get on with yourself
EDIT: Take things slowly. Girls don't like to be rushed.