Magical Girl Apocalypse

Mahou Shoujo of the End (魔法少女・オブ・ジ・エンド)

Plot
Kogami Kii hates his normal everyday life and wishes for something to happen. One day in the middle of class, he looks out the window and sees his teacher’s head explode when a little girl hits it. He goes to the bathroom to calm himself, but when he gets back to the classroom, he finds most of his classmates killed by the same little girl.