If you're shy and nervous about being rejected or embarrassed, just give her a call. That way you will have more control over the situation since there will be less variables to contend with (people around, looking nervous or blushing, etc). Call her up and say "hey, this is (your name)... I just wondered if you don't already have a date for the dance, if you'd like to go with me?". Simple as pie. If you just be direct about it, there's less chance you could ramble on nervously or say something stupid.