Rabbit Heart(1987)

The thirteen year old Janni is depressed, because she's still a little behind in her physical development. This leads to bullying remarks from her classmates. One day, a film crew comes to her school to find a candidate for the role of a Prince in a movie. They choose Janni, who they think is a boy. She takes on the role. At school, she tells her classmates she plays a Princess. When the moment of truth approaches she must find the self-confidence to invite the class to the premiere.