Willy/Milly.

Milly is annoyed whenever her mom tells her to behave more appropriate for a girl. She's just not interested in fancy dresses or prom dances. Some day her best friend's little brother offers her a potion which can make her deepest wish come true during a solar eclipse. She goes through with it and ends up with male genitals, additionally. So she has to decide if she wants to live as a boy or a girl. Her father, who always wanted a son, supports her in checking out life as a boy. Hence Milly changes school and starts out as Willy.