This doesn't seem ambiguous. "All you guys are" seems to be narrowing the focus of the sentence, because otherwise it would be more fluidly spoken as "all of you are", and it sounds unnatural to add to the sentence for no other purpose. (It sounds somewhat unnatural under either interpretation, though). If it had been "you guys are all", perhaps that would be ambiguous, but only with a strong emphasis on the word 'guys', which is not how the phrase is normally spoken. Either way I'd expect the dance teacher to be using hand gestures at the same time to indicate which people they're giving directions to.