Monogatari no Naka no Hito

A young noblewoman once dreamed of becoming a magic user like in the old tales, but her time in magic school has dimmed her passion. One day, a mysterious man comes to her rescue in the forest, using magic like in the old stories. Despite the fact that he is clearly skilled in magic, he is thrilled by the idea of learning new techniques. Since the only way to do that in this day and age is in magic school, he alters his appearance to school-age and decides to accompany her back to school!

(Source: MangaHelpers)