Plot hole: Even though Rita left the choir early into rehearsal for the contest, and her mother took her music away to prevent her practising before she had learned it, she still manages to know the song by heart and the dance routine perfectly when she rejoins her friends last minute. She couldn't have perfected her performance in the time she had.
Suggested correction: First off, I think it's possible for them to practice the song as they're going to the performance and second, the dance routine looked more improvised and considering that they changed the arrangement at last minute, I think it makes sense. Also, who's to say that Rita doesn't have a good memory?
On the same note, they repeated the same moves a couple times. She probably just caught on.