So the one behind Nishida Suicede is the male teacher. The one who killed the female lead was Fukouka ,it seems the female lead somehow found the truth about her death in her timeline, she even kinda asked mc help but he was not interested or something
and when she walking home, Fukouka stab her. Fukouka was gay and have feelings towards the male teacher and seems try to protect him. I think Nishida was pregnant or something, the male teacher was quite a bastard, i don't think he reject her love and make her suicede, the pregnant part was just my theory, either way i think he do something nasty on her and it drive her to suicede. The one who killed mc best friend seems the Hori girl, she try to kill him again but got stopped by mc and his friends. She also love the teacher and try to protect that scum. The ending is the fukouka guy realized he was used by the teacher, try to stab him but got kicked by mc best friend, police come, both Fukouka and the male teacher get arrested by police. In the end the female lead ghost dissappeared but she merged with the current female lead who still alive, they then confess each other love, kiss and the end happy ending.