When you send your child to a private school, you do so because that school offers a better education than does a public school. One of the ways that they do that is by controlling the environment through different rules than are present in public schools. You agree to those rules when you enroll, and this is especially true of religious based private schools.
So for a female student to become outraged and complain when a Christian private school requires her to wear a dress in order to attend a school sponsored prom is a bit disingenuous. It would make as much sense if she was outraged when asked to pray before class. You can always go to a different school.