Well the truth . Or if you think she wont have a date and wont go to the dance you could lie but then there's her friends finding out and blah blah.
I once had a date to a dance and the day b4 the guys wouldn't return my calls or anything . So I had to call and ex to fill in and I saw him at the dance w/ another girl but I didn't get mad or anything.
So if you asked Anna you must not b serious abt this other girl so she should understand.
Just say, "Hey, you might want to find another date, I'm going with someone else, sorry." Don't make her wait so she won't be able to find someone else.