Hidan no Aria (Japanese) - Aria The Scarlet Ammo (English)
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.
One of my fave anime. Warning though. Contains some ecchi.
You can compare it to Highschool of the Dead.
I have already read the Manga and Light Novel of this series.