UQ Holder!

UQ Holder! (ユーキュー ホルダー) is a manga written and illustrated by Ken Akamatsu.

Plot
Touta, the main protagonist., was killed when he was a little kid and his parents begged Yukihime, a vampire to save him by making him a vampire too. After that, she raised him for two years as he had no relatives and now she considers him her family. As a teen, Touta and his friends decided that they will surpass the incredibly strong Yukihime, who is their teacher and attacked her every but they were always beaten up. But someday, another teacher offered to teach the kids magic in order to defeat her as she was using magic too. They accepted and the next day attacked her. She was unable to move since Touta gave her a bracelet that wouldn't let her use magic. But before the kids manage to defeat her, the other teacher showed up and K.O.ed all of them in an instant. It was later revealed that the teacher is actually a bounty hunder aiming for Yukihime that holds a bounty of 600M yen. And the story continues.