yea ask someone else. if she was serious about the "ill think about it" she would have either called you that night or told you the next day to tell you. most of the time this is an excuse. if you go ask her like a whimp "so did you think about it", she will just have you on her leash.
heres what you should do.
1. go find another girl to go to the dance with.
2. go back to the girl that told you she would think about it and tell her you have found someone else to go with.
DONE. not only will this make her think if she is NOT good enough for you, she will start to wonder what this girl has that she doesnt. make sure you dont tell her this prior to actually asking another girl, or else you might look like a fool if things dont turn out the way you planned.
at all costs, apply C+F if she asks you who it is, and never give direct answers.
example:
her: oh cool. who are you going with?
you: im going with a very...... (pause 2-3 seconds) ....... sexy girl. (smile)
most probably she will try to squeeze it out of you.
her: whats her name?
you: arent you the nosey one. ill tell you what, wait till the formal, and you will see her.
her: BLA BLA BLA
you: well listen i got to go. bye
her ego + confidence DOWN!