Between 18-19 (the drunk scene you mentioned), he raped Chiwoo. In chapter 14, he forces himself onto Chiwoo which is again fucked up but that's not really the chapter I meant. In chapter 14, you can clearly see Chiwoo recalling that drunk incident (he was probably thinking whether Jihyun would rape him again or not). There are hints about the trauma Chiwoo faced although I don't remember what. When I read the story, I knew right away after that incident that Jihyun raped Chiwoo but I don't %100 remember what made me realise so quickly since its been a while. It wasn't depicted clearly so I guess it's normal you got confused. The best evidence I can give you is this: So, maybe you know it but there is kinda like a sequel for Back To School written by the same author: It's called: “Back to School: All Grown Up” in that, just go to the author note and read it. The author states that Jihyun committed a sexual crime and that Chiwoo had trauma from it (it says crime and I’m pretty sure Chiwoo didn't have the trauma from a kiss) which is why they'd never end up together anyway.
I absolutely loved how happy Chiwoo and Kyujin ended up! but I honestly wish Jihyun had gotten some kind of happy ending. Like i know he was a terrible person and did stupid things, but he went through some stuff with his dad and he was also just a kid back then and i feel like in his mind the only way to be happy was by being with Chiwoo and he just went about everything the wrong way. I wish he would have gotten another love interest or something bc when he cried in the car it was just soooooo sad :(