I have a 9 yr old that takes dance (though not ballet). I disagree that hiding the leotard necessarily means she really doesn't like ballet or something else is going on. It's possible and I definitely would investigate, but I think the more likely scenario is that she likes whatever else she's doing that moment more than the idea of going to dance class, like rubidoux said about her son's karate class.
I might let my girl skip a class, but I wouldn't let her quit altogether unless there was something else going on (like a mean teacher). Like rubidoux we also don't do much punishment in our house. Assuming there's not a major issue with the teacher or something else, I'd tell my kid that I have paid for the class and they need to pay me back if they're going to drop it especially after we had a big discussion before signing up about whether she wanted to take it or not. I'd let them quit if they paid me back. I view that as a natural consequence, but it might feel punitive to some. My kids get an allowance, though, and it would be part of our learning about money. I don't directly buy them toys or candy, etc, usually either (unless it's a birthday or other special occasion)— they pay for that out of their allowance money so paying back for the unused classes would make sense to them. However, we pay for our dance classes by the month only, so I would be willing to let them quit after what we had paid for had passed. I am also willing to let them skip class if they feel sick or have a heavy homework load or maybe even if they're just having a super crappy day. Maybe this is also a natural consequence for you to learn not to pay for the whole session in advance :) .
That said, my kids don't do ballet. I know it's not a great fit for them. My dd1 thought she might like to take it and she really wanted me to sign her up, but I put her off by letting her take one free trial class and told her if she liked that we could think about signing up for it. She didn't even make it through the whole class before she was coming to me saying she didn't like it. I did have her finish that one class (55 minute class) because I thought it would be rude to walk out in the middle, but we did not sign up for ballet. She's more of a "contemporary" girl.
Now, as far as the lying, we don't do punishments, but we would have a Big Talk about that and I think the advice upthread about talking about trust with her is solid. I don't think I would buy another leotard since I'd already feel like I was wasting money on the ballet class, but I would keep the leotard and not give it back to her until it was time for dance class next week, so it couldn't get "lost" again.
I would then try to work out a mutually agreeable solution ala Ross Greene's Collaborative Problem Solving approach or Faber and Mazlisch ("How To Talk So Kids Will Listen And Listen So Kids Will Talk"). I would lay out your observations about the dance situation — 1) she says she doesn't want to go, but after class she reports that it was a little bit fun, 2) you have already paid $$ for these classes, 3) she agreed beforehand that she really wanted to take ballet again, and then I would ask for her input. What is it about dance class that she doesn't like? You might offer a few suggestions — is the teacher too strict? is your dd tired after school? are some of the other kids not nice? does your dd want to do something else then instead (play, watch a video, etc). And then when you have gotten to the root of the problem (the teacher's not mean, but it smells really bad in there; or I just wanna stay home and play Minecraft or some of the other girls make fun of me) you can define the problem, write it down and brainstorm for some mutually agreeable solutions. We have used this technique many times and it really helps to show the child that I am taking her seriously (you write things down after all). It also helps to really define the problem and help us both think all the way through it. Sometimes the kids come up with great ideas. They also like to sometimes come up with very silly ideas, also, but I write them down, too. Going through the process of putting it all down on paper makes it really feel like a Big Deal and that you are taking both the dance class problem and the lying very seriously. You may find that the problem is something more serious like girl bullying or it may be something more trivial (wanting to play My Little Ponies instead). Really getting to the root of the problem will help you decide whether to insist that she finish the session and it will improve the trust between you.
If you like you can also do a separate brainstorming session about the lying/sneaky behavior. You may find that she has some good suggestions for solutions here too.
Edited by beanma - 10/26/13 at 7:59pm