Houkago

The story focuses on Maejima, a teacher at Seika Girls Academy and an adviser to the archery club, feels that he has recently been targeted by someone. Maejima also consults with the principal, who dismisses the situation, except for the archery club members, Yoko Takahara and Keiko Sugita, who harbor their respective feelings for Maejima.

During practice day however, the guidance counselor Murahashi was found killed by cyanide poisoning in the teacher's changing room. It was a complete locked room murder, where the men's locker room was hooked up from the inside, and the women's locker room next door, accessible from the scene, was locked. As the search for the culprit who wants Maejima's life continues, a second murder happens during the sports festival.

(Source: Seika Joshigakuen Fandom Wiki)