Kill Me, Heal Me!! I've only watched 2 episodes but my god it's so good. (THANKS AGAIN SONGYI!!) Even though I dropped it, I'd still recommend Who Are You: School 2015. It was a very good watch for me up until episode 10 and I just couldn't get into it anymore.
@Joy School 2013 is (in my opinion) one of the best school dramas out there! I would recommend it much, much more than the other versions (2015 + 2017). I'm glad they didn't focus on romance but instead on the class (that consists of classmates with different problems). They also focused on bullying quite well.
@momo, I rather did enjoy the 2013 version. I think it tackled high school problems quite well and I really enjoyed watching the friendship with Kim Woo Bin and Lee Jong-suk unfold. Also, I really like Jang Na-ra and thought she was really good in this show.
@Sandysan You should really watch Master's sun. First ep or two are a bit scary but the rest are fine and it's pretty funny and entertaining and So Ji Sub's acting in it is 10/10