Girl 1 = main character would be the invisible girl, and she would be almost all the scenes.
Girl 2 = main supporting character would be the friend who would come in two thirds of the way through the film. Her role would be to teach the main character that its ok to be different, and you can always find a friend anywhere you go.
Classmates = I would also cast three or four girls to be the main students that ignore the invisible girl. I would also have other students, because this is a school and there aren’t five students in the whole school.