I used to make an almost invisible bra using an upside down pair of tights - waistband becomes bra strap, and the tights are cut to fit around the bust and perfectly under leotard straps. It’s not super supportive, but it’s a huge improvement on no support at all, as well as comfortable and also somewhat minimizing. Worth playing around with some old tights to see if she can fashion something that looks and feels good.
Wearmoi also makes a bra that is as thin as tights with clear straps that might fit perfectly under a leotard. I doubt the teacher would even notice she’s wearing it.
Bigger picture, the school not allowing a bra and not being supportive and understanding of normal childhood development, does seem like a red flag. There are lots of wonderful teachers, and I would definitely take this opportunity to switch to a school that sees the beauty and potential in your daughter. I would also be honest with your daughter that while the training at her current school may be excellent, they are certainly not the end-all-be-all of schools because of their rigid, old-fashioned thinking about body types and probably many other things. I always remind my children that we will change schools at any point if that school is not giving them what they need.