It's in ch. 19. They were with Chiwoo and Jihyun were out with some guys drinking and Jihyun got drunk and started to hit on Chiwoo again, so Chiwoo asked the guys to go away. Then there was the argument between Chiwoo and Jihyun and when Chiwoo left and Jihyun went for a smoke, he met one of the guys (with whom they were drinking before) that killed the student. When he saw the dead body, someone walked in on him and from there everyone thought that Jihyun is the Killer. And when they were interrogating the group of guys that Ch+J were drinking with, they all Said that they were together, and because Chiwoo was in hospital, he couldn't help Jihyun in any way (also, Jihyun was known as the bad kid, so they all assumed that he just did it, cuz why not i guess) I tříd my Best, so sorry if it's not understandable ヾ(❀╹◡╹)ノ~

I have a question, so who actually killed the student if it wasn’t jihyun? I’m sorry if i missed a scene ╥﹏╥