Ikonik Angel
It's just could've or must've. You don't have to write out have all the time, 'cause that's not how it's said. In formal writing though, it's probably better.
That 'I wish I were in a dance class' one just sounds and looks rather wrong though... Oh? Well, I'm not the best grammar girl. I'll change it to 'I wish I were you'. Does that sound better?
Remember: Just because it doesn't sound right, doesn't mean it's incorrect. The way we speak on a daily basis and the rules of grammar tend me be a lot different for 99% of the population.