I think having Kelly perform at the dance would be the easiest way of bringing her into the game, and at the dance announce or have the faculty announce that she will be becoming a student there, like:
"And now, for your listening enjoyment we have booked a singer that you most likely all have heard of, and who will be joining us here for the remainder of the year as a student, Miss Kelly Mitchell!"
Well what do you think? Just an idea that could fit. Kelly of course being embarresed at being singled out like this when she just wants to fit in and put the whole popstar thing on the back burner.