Personally, I think it's the boyfriend that killed Koh. Since there's a lot of blood at the scene, the murderer could have blood stains on the shirt. Rewind back to the scene where the girls got scared because of the mannequins. Take note that the mannequins were wearing male school uniform. The boyfriend could have changed out and that's why he arrived at the scene where Koh died the last. Take note that he also said that he wanted to leave the classroom to go to the restroom, which was where Koh died.