We all know that Yun Ho is the killer but it is really fun to imagine the idea that Yun Ho said "K is actually Yun Ho". Just a detective chasing someone and realizing in the end that it was his split personality or something
if SY/K really killed yunho's father then that is honestly sad since at the end of the drama he would have to…
SY won't go to jail because he was a minor at the time. He probably will be suspended from the police though. As to why YunHo didn't kill SY is probably because he felt a connection... like they were the only ones that could understand each other's pain. And if SY/K did kill YunHo's father, then there is more reason not to kill SY because from YunHo's perspective, SY/K put an end to YunHo's "misery"
One of the main aspect that I have come to love about the school series (2013/15/17) was how close each character felt whether they are main or side characters. This series had the same aspect. Plot wise (apart from the romance and the lawsuit) this was exactly school 2013 and 2017 (some dialogues as well). But I do love school 2013 and so I loved school 2021 as well