Onigokko!

A long long time ago, the oni (demons) collected many treasures on an island known as Onigashima and Momotarou came with his three friends to defeat the oni and recover the treasures. Now, there is a thief Ura that is said to be the descendant of the oni and who is stealing all the treasures.
His actual name is Keisuke, the protagonist of this story. However, someone else posing as him sent out a notice and he wonders who it is. But soon his cover is blown as the descendants of Momotarou, Kintarou, Issun-boushi and Otohime also attend Onigashima Gakuen. And thus a modern fairy tale is born.
(Source: Hau~ Omochikaeri!)