School Ghost (1987)

Carola is a mischievous girl who doesn't care much for school- except for sports and recess, of course. Without her good friend Willi to keep her on the straight and narrow, she would really be in trouble. One day at school, Carola has an idea. She invents what she calls "International Ghosts' Day" and a ghost named "Buh" to go with it. When Buh turns out to be less-than-imaginary the two decide to switch places, with Buh taking on all the schoolwork, and Carola taking the opportunity to play practical jokes on all her friends...