Negima! Magister Negi Magi(2003)

Mahou Sensei Negima!

After graduated from a magic academy, 10-year-old genius boy Negi Springfield was assigned to a huge Japanese school as an English teacher for practical training. To his surprise, the 8th grade class assigned for him is all-girl. In addition to teaching (and being teased by) those 31 pretty girls while trying to keep his magic capability in secret, he's also looking for clues about his father, who was once known as "Thousand Master" but mysteriously disappeared years ago.

(Source: Anime News Network)