Negima! - Ken Akamatsu

Ten-year-old magical prodigy and girls' school instructor Negi Springfield has a new secret... and some of his more stubborn students are determined to uncover it. Few know that Negi has magical powers. (In fact, he'll be turned into a weasel if he is officially found out.) No wonder Negi is so cautious while taking magic lessons from the so-good-at-being-bad Evangeline. As one of the few students who actually know about his special abilities, Asuna is the one girl to whom Negi decides to tell another secret - one involving Negi's long-lost father, who disappeared years ago. But what connection can there possibly be between the Thousand Master and Asuna?