Maburaho

His name is Kazuki Shikimori, 17 years old. He attends Aoi Academy, a school for elite magicians. Each magician has a set limit of spells, and performing more spells than the limit allows will cause the user to turn into dust. In this case, the number of magic Kazuki can use is far below the average in his school. Then one day, all of a sudden, three girls: Yuuna Miyama, Kuriko Kazetsubaki and Rin Kamishiro, appear before him. But of all the things that they are after, it is his genes. And for Kazuki, the girls' appearances may also spell his doom, as he soon realizes that his magic count is decreasing ever steadily due to the fact that he can't resist using magic to help them.

(Source: Anime News Network)