R.F.
K.,
For whatever reason, she just doesn't want to participate in the actual class. It sounds like she enjoys getting ready for class and the anticipation of starting class, just not the actual class. My advice, and just FYI -- my 21 year old daughter danced ballet & jazz since she was 3 (she was also in a ballet company); so, I have some experience in this -- don't make her attend class. If she changes her mind and wants to go in, she'll let you know. Then you can visit with her about what it's going to take behavior-wise from her to be allowed in class. But, she may never decide to join the class and want to do something else. I applaude your efforts to expose the girls to various things in order for them to have options from which to choose in the future. However, ballet simply may not be this one's cup of tea.
Best wishes,
R. F.