It's totally fine for a high school to do. If children's theatres around the country are doing it, it's most definitely appropriate for a high school. If anything I would say it's inappropriate because high schoolers won't want to come to see a musical about the ugly duckling, not because it's too racy.
Eeeeeeyyyyyyyyaaaaaaaannnnnddddd aaaaaaaiiiiiiiiyyyyyyaaaaaammmmmmmm teeeeeeeelllllliiiiiinnngg yyyyooooooouuuuuuuwwwaaaahh...