It sounds like your daughter is perfectly capable in theatre, and they are not giving her a chance. They sound like flakes. Cruel flakes, to be honest.
I believe that Exgineer brings up a good point, though. Many parents think that their darling child is the most talented, wonderful person ever. And it can really be damaging (in the long run) to the darling child, because sooner or later (no matter how talented they are—or not) the bubble will burst, and they will be told that they are not “all that”. However, in your daughter’s case, it sounds like you have a full grasp (a very healthy grasp) of her strengths and talents. Good for you. And I think your obvious grasp of your daughter’s abilities (in a realistic but supportive way) have helped her. She’s obviously a “class act”, and no doubt you’ve helped her be that way. (Just my guess.)
It sounds like she’s perfectly capable of doing these parts, and they are screwing her over for whatever reason. And it’s not right. However, that seems to be high school. It’s all screwed up, and nothing makes sense. Let me repeat that: NOTHING MAKES SENSE. Or maybe that’s just my own prejudices about high school stepping in. 
I just remember my own high school experience. Maybe this doesn’t relate so much to theatre, since it is about art, but I have a feeling that the dynamics between the two are similar, especially when it comes to high school.
In high school, I was always keenly interested in art, and I was considered pretty good at it. (I sold my portraits, did comissions, blah blah blah.) But like even sven and her experiences in high school, I was passed over for opportunities repeatedly. Broke my heart more than a few times. Maybe there was something in me that was getting me passed over, but whatever it was, it wasn’t because my ability was inferior to those who got the breaks. And it seemed to me at the time that some of the people that were chosen for the opportunities often didn’t really give a damn about art that much (it was more of a “lark” to them). In some cases, they weren’t very good artists at all. It made no sense, at the time. It still doesn’t. It was almost as if I had a sign painted on my forehead saying, “No matter how hard she tries, just ignore her.”
But it was a long time ago. All I know is that everything changed after high school. It seemed like things evened out a little more, made more sense. The untalented prima donnas weren’t catered to and didn’t get all the opportunities anymore. Those of us who cared a lot and worked hard seemed to be recognized for that, at least a little more often.
Man. High school sucks. That’s the only pearl of wisdom I can offer.