Hidan no Aria OVA

The story takes place in Tokyo Butei High School, a special school where armed detectives — “Butei” — are trained to use weapons. Kinji Tooyama is a second-year-student who has a special ability, but he keeps it a secret to maintain an ordinary, peaceful life. However, when he gets caught in a bombing on the way to school, he encounters H. Aria Kanzaki, the most powerful S-Rank Butei student in Assault Studies.