(SMALL SPOILER)

Can you tell us that does Yohan know about Jun's crush on Junwoo? And if he does, what does he think about it? Why does he say that being gay is disturbing to Jun? And why is Yohan friends with Yohan in the first place and why do they fight?

he does and he also knows that jun was hit by junwoo twice (he lives next door to jun, so he saw both the day jun was hit at school and at home).
yohan's mad jun likes junwoo and not him. and it's hard to explain but basically yohan thinks jun likes junwoo because junwoo is at the top of the school hierarchy, so he's taking the chance to take junwoo's place at the top (though he doesn't care about other what people think of him, only jun) through the rumours of junwoo being gay and encouraging people to bully junwoo. he's not homophobe, yohan's family knows he's gay.
also yohan dislikes junwoo and thinks of himself as not his friend. he only got close to junwoo because jun is always hanging around junwoo. he also always treated junwoo bad and repreended him for drinking/smoking/sleeping around, but everyone always thought yohan was being ironic with the religion thing and laughed it off as his jokes. he also hates junwoo bcs he knows jun likes junwoo (same way jun hates taesan)
the fight occurs because when junwoo comes back to school (i think he and taesan disappear for around two weeks after the fight), some kids threw away his school books. yohan starts joking around about junwoo never studying so he didn't need it. junwoo ignored yohan's comments and starts accusing jun of stealing his books. he throws his backpack on jun and that's when yohan starts to provoke him talking shit about taesan (he says something about junwoo treating everyone's ears like they are taesans loose asshole and thinking he can do whatever he wants with it). then, junwoo loses it and attacks yohan. yohan gets hit a few times (... because he wanted to, really) then pushes junwoo away and gets up. the class watches in horror yohan pulling his own tooth that became loose with the punch. jun makes one of the classmates get milk and goes after the fallen tooth so that yohan can reattach later, meanwhile yohan fucking destroys junwoo. when junwoo is almost dead and unconscious, he gets a metal chair and hits junwoo's face with it. then the ambulance comes, junwoo is taken away and yohan calmly walks towards the ambulance while covered in blood. jun gives him the tooth and yohan carress his cheeks, then goes to the hospital. the teachers even ask if junwoo was not ganged up from how brutal it was. later, jun and yohan visit junwoo at the hospital to like. spit on his face and take some pics of him unconscious. then yohan takes junwoo's place at the top of the school hierarchy and junwoo leaves school after a while.

Wow! That's intense. I'm so glad about what happened to Junwoo and more glad that Jun gets over him. Thank you for taking the time out to write such a detailed response.
Can I also ask whether Junwoo father getting arrested was Yohan's doing or not and what do Taesan and Junwoo do when they are absent together from school for long durations periodically?

hmmm i dont think yohan was the one behind the arrest. he tells jun that junwoo's father was having problems with the law and that junwoo's uncle was taking over the whole company and jun thinks that it was yohan behind it and asks, but yohan says he only heard about it and that soon everyone'd know anyways. if that was a lie and was later explored, i might've missed it, but i don't think yohan'd have any direct power to arrest him.

also it's not said where or what taesan and junwoo were doing, just that junwoo kept dragging him around the city and some people saw them. tbh i found it very weird how they were considered the secondary couple when the manhwa was announced since they are really just background characters that stop appearing after the first act... there isn't much to say about them, just that junwoo leaves him alone/drops out and taesan goes on with his life (and since he's afraid of yohan he barely speaks with jun after they enter third grade)

Wow! Ok. It seems that that I have to sit through the initial angst to get the good part later. Thank you so much for the spoilers. I can happily read the story now and wait for Yohan's crazy side to come out.