The story line is very interesting and outside the usual KDrama standards - the characters, while some seem to be better defined with some growth potential, many are just cursory and their story is just muffled even when it should have been an important one - many missed opportunities in my opinion. If it will be just for the general story, directorship, editing and some of the supporting actors, this would be a 5 stars - what makes this a generous 3 stars are the ML and FL: the actors chosen, the dynamics and chemistry among these two and the missed opportunity in some important characters and story development.
1. the ML and FL ( Chi Yeol and Haeng Seon) have absolutely no chemistry so even attempting to portray something romantic looks actually very misplaced and fake and unpleasant to watch - the actors, individually are good but together as romantic partners, absolutely a mismatch - this was a very poor choice on the casting - the FL is supposed to be some years younger than the ML but the actors chosen are in total reverse by a significant gap in years and contrary to other instances, this gap in years really shows and is contradictory to the story line and then, add the lack of chemistry between the two, it makes a really poor choice in casting. This would be a 1 star at best just because negative stars are not available.
2. Some of the supporting characters are brilliant - the autistic uncle, the two vocal mothers from the 'mothers chat' (Su-He and Mi-Ok), the two male friends and classmates, the daughters (Nam Hae-yi, Dan-ji, Soo-Ah) are really brilliant and they make the series worth watching - this will be a 4 star but unfortunately they are not the main story
3. the story line gets confusing at some point and remains sort of murky all the way - the metal ball killer, motives and how it got there and then he jumps off the building .... to me, how someone gets to the point of killing is important and how it then gets to jump off the building is also important so not sure why this ark was rushed and was so muddled - this is 2 stars at best - maybe some one thought to be more important to play, and play, and play the misplaced romantic FL and ML instead of some character development related to the general story like the far reaching obsession to 'make it' to the right university, be the top score in class, etc, etc.
4. a short cameo by Kim Mi-kyung, one of the best 'mothers' in the the Kdramas - this time she plays the young FL mother that was the 'savior' for the ML.
In the end, yes, the series is worth watching because it touches on some important topics like the obsession in SK culture in scoring high on tests to the detriment of actual education and learning, the stress and negative impact this obsession has on everything that it touches, the students, family, relationships, morality and ethics.