Is it normal to have teachers helping girls change costumes?

I have two beautiful young daughters that go to high school. My daughter C is 18 and a senior and my younger daughter L just turned 15 who is a freshmen. Both of our daughters are in the varsity show choir for their school. During their performance they must do a costume change which means they have assistants to help them change. I'm very uncomfortable with a 27 and 34 year old assistants helping young girls change. I know it's a professional thing and all but some of these girls are so young and during the actual performances the girls wear bloomers over their thongs, but at practices they don't have bloomers and all these girls are getting out of one dress and into another very quickly. I'm a very concerned mother knowing these assistants see all the girls in their performance thongs. My older daughter C says it's just something you get use to as a freshman. I wanted to just not worry about it until last night L came home from practice crying saying that during the costume change a zipper was caught on her full coverage performance bra and ripped down the middle and fell off right as the guy performers were coming off stage and everybody saw L's breasts. L was too embarrassed to go to practice today even with C trying to cheer her up saying how that's happened to other girls this year including herself. The only difference is L is much more shy and younger than C and is far to young to have those assistants accidentally see her private areas, my husband is furious and wants the girls out but what should I do? Let then stay or have them be done with show choir?