I agree that the song choice is a little bit
. At first I thought "it's just a happy-ish old song, no biggie with that" but then, actually, as I think about it, what should 12-13yo really know about it raining men?! If it was raining mobile phones, or Creme Eggs, or makeup, then great. but men?! It's hard to interpret that in a way that isn't sleazy for anyone under 18. When Geri did it, the video had a sporty Fame feel, is that what the dance teacher is aiming for maybe? If so she would have been better to just do the number from Fame perhaps hey 
I think you are justified in having a conversation about it with her. I wouldn't stand for any "how dare you question my teaching" crap. I would say "nobody's questioning your teaching, but I am questioning your judgement on what is appropriate for 12-13 yo". It would be helpful if you could name some of the other mums who agree that the subject matter isn't appropriate along with any bump'n'grind moves.
If she says she doesn't expect to have to court opinions before choosing suitable material for her class, I would say "that's your perogative but then you must expect opinions to come afterwards especially when the subject matter is based on sex no matter how it's been dressed up in a spangly leotard and legwarmers".
She should be told when it's gone off track every now and then. As long as you keep your cool it should be fine. Don't forget the compliment sandwich "Ms X, you know (DD) has a fantastic time learning with you and her dancing has really improved over X months. However I do have some concerns over the latest number you have chosen for the girls. (etc). However she does really enjoy it here."
GL!