Willy/Milly (1986)

Milly is annoyed whenever her mom tells her to behave more appropriately for a girl. She's just not interested in fancy dresses, or prom dances. One day, her best friend's little brother offers her a potion which can make her deepest wish come true during a solar eclipse. She tries it and becomes a boy. Now he has to decide if he wants to live life as a boy, or return to being a girl. His father, who always wanted a son, supports him checking out life as a boy. Hence, he changes schools and begins again as Willy.