Hidan no AriaAria the Scarlet Ammo - Akamatsu, Chuugaku Kobuichi

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)

Hidan no Aria has been published digitally in English as Aria the Scarlet Ammo by Digital Manga Publishing since September 30, 2013, though only two volumes have been released so far.