Can you call him? Text him? Send him a message on MySpace?..
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Don't overdo it: just a quick, "Oh, FYI, I wanted you to know that the dance invitation is No Strings Attached. I'm not looking for a relationship right now. I just want us to be able to have fun without any pressure!" ..
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Edit as necessary for minimum texting charges...

I wouldn't talk to him about it because then it would seem like you really do want to be his girlfriend and you are writing the note in case he rejects you as that. I would say that you should just go to the dance and have a good time and if he brings up something about you being his girlfriend tell him that you don't want a relationship in high school and you want to remain friends. But if he doesn't bring up relationship than I don't think you should since you don't want one anyway...

Yeah, I'd say just play it cool. Notes always have the potential of being awkward, even if they seem like the easy way to communicate. Just don't act like you want him to be your bf, and he has nothing to worry about. No big deal, you're going as friends. Maybe you can even call him "dude" or "bro" or "man" so he doesn't think you're too soft on him...

Geez, I wouldn't put that in a note. I think that's the type of note that could be passed around to everyone and totally humiliate you...
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If he said yes, just go to the dance. If he doesn't talk to you very often and doesn't call you, then I don't think it matters if he thinks you want to be his girlfriend, because it's clear that he isn't trying to make you his girlfriend. Just go to the dance and have fun. Smile at him. Talk to him when you see him. But don't start baking him cookies and offering to do his math homework for him...