Just a little food for thought as far as letting her "sweat it out"..... I see all the pro's and con's of the dilemma and - (as a mother of FIVE who's so far survived three teenagers - 2 of them girls - and still working on a 12 year old girl and 10 year old boy) - I would have to say.... "KEEP YOUR FOOT DOWN!!" - - trust that you'll be kicking yourself with it later if you don't!!
It's hard to enforce your word if they know they can weaken you even once!! haha!!
BUT - - in the event you do decide to "let her slide" - (and not knowing her personality, she very well may be the type of kiddo that only needs a good scare to straighten her up... this could have been it!) - and you do let her go to the dance..... the "sweat it out" thing could come back and "bite you in the bum" so to speak....
If she truly thinks she's messed up and isn't going to be able to go.... she'll of course, cry to the boyfriend... who likely will in turn, ask someone else as soon as he can!! Then you'll tell your daughter she can go.... and she'll "hate" you regardless because now she'll be going solo!!! haha..... (I don't mean to laugh... but I have SO been there!!!)
Poor thing.... I feel for you!!! Know that the hatred rarely lasts through to the next allowance due to them!!!
<big hugs> Good luck!!
~ Dena & Natalie ~