For the time being I want to pronounce a spoiler warning. I will disclose the identity of the murderer below.
In the first time line (so before the main character has prevented the murder), the teacher has murdered a pair of his students. But this one boy who looks like a girl was also murdered. It was also mentioned that the killer has probably held him for a girl (as the perpetrator has only on girls apart). Of course, the teacher knows his students. He would never have been able to hold him for a girl. Why should he murder him?
No, he has deliberately murdered Hiromi to assume that the perpetrator does not know the child and thus only accidentally caught a boy. Basically, the perpetrator no matter who is the victim, the main thing is alone.
He did not recognize that it is his student or so
Maybe he fell briefly on the trap .... yes; -;