Hidan no Aria

Tokyo Butei High school—It was a special school where armed detectives, also known as "Butei" are being brought up. Tooyama Kinji is a sophomore student who has the unique body which activates a "super mode" when it was stimulated by "something." He tries to hide it to others in order to live a peaceful life. But trouble came to find him when he became a victim of a bombing incident, where he met the Butei high "Assault" department elite, Aria H. Kanzaki.

(Source: Baka-Tsuki)