I have read with great interest all the posts on this thread. I see it from both perspectives, so I'd like to give a little insight into my experiences. First let me say I am the mother of two professional daughters, both of whom have had their share of professional parts. I am also a music teacher (elementary private school and give private voice and piano lessons). So, I feel I have some right to give my opinions on this subject.
There is nothing worse to a parent of a professional child than to see that child overlooked because he/she is "too good". Both my girls have had to deal with this. In professional theater worlds, they have both been cast as leads and/or supporting roles, done a lot of solo work in many concerts in professional vocal troupes and always been the one with solos in dance concerts. Why then, can't they get cast in high school musicals, other than supporting and chorus roles? It's been a frustrating question and one that I've had to think long and hard about. And, believe me when I tell you I've shed many tears along with my children over not getting cast when obviously less talented students have been given opportunities.
But, the teacher in me knows that sometimes we cast students who are perhaps, "less talented" because it IS school and should be a learning experience. Several of my colleagues feel that when professional children come to auditions, they intimidate other students, taking away the "learning" aspect of school theater. I have to agree. Several times I've heard my children's friends say it wasn't worth auditioning because "she's so good, why waste my time?", and the point is, it IS school theater. And, most school directors are mere teachers who, like Julie said, may be intimated themselves by the talented student who has the professional credits.
My advice for any student who has experienced this is to just enjoy high school theater for what it's worth......camraderie among friends and an opportunity to work on a group project. I've found that, after the tears and heartache of disappoinment after the cast list goes up, my own children have enjoyed being with friends and enjoying the show. And, if you are unable to overcome this kind of disappointment, then realize how the high school casting process is done, and don't audition, and find something that fills your own needs, i.e., community or regional theater.