Wait, this is just wild guess.
But what if the one who killed the delinquent was the girl from the beginning. I mean, there was no relationship between the suspected killer student and the girl. YET, he knows her private love life. (Even the MC didn't know)
And the secret messages, it should be someone Suspected Killer is close to, someone who already knew the killing (in other words, the killer). Though, I'm not sure about this.
Welp, I'm really looking forward to this forward to this manga. 😃