Kouto Onmyou Gakkou: Kizumono no Hanayome Gaiden

The Imperial Capital Onmyo School—a three-year, full-boarding school that trains members for the Imperial Onmyo Bureau, a national organization dedicated to exorcising demons. Yakou, Kazunari, and Reiji are second-year students at this school. One day, Reiji is in danger of being expelled after injuring the son of a military official. Yakou and Kazunari try to help Reiji by appealing to the headmaster, but in exchange for rescinding his expulsion, they are ordered to investigate and capture a mysterious masked man named Semimaru who is attacking the upper class.

(Source: Kodansha, translated)