Maten Gakuen ~Jigoku no Love Love Daisakusen~

The tribe of demons dwells above the Himalaya mountains. The demon king Zenon is harboring a plan to occupy the Earth and subdue the humans under his yoke. But first, following a demonic ritual, he sends his son to a Japanese private high school. With his side-kick Zabo, who can miraculously change forms and genders, the young demon offspring is incarnated in a human form and has to deal with the hormonal problems most human teenagers suffer from.

[From Moby Games]