First of all I'll just go ahead and tackle the big elephant in the room with this drama: FL is misscast.
I have nothing against the actress and she looks great in her age, she just doesn't look the age she's supposed to be in this drama and I find it insulting I'm to believe she's around her mid to late 30s. Episode 8 clearly mentions 2010 as the year the mother passed, at that point ML was in university and she was in the team. EVEN IF I assume FL to be 26 at the time, that would make her around 39 years old in present time (and that's a stretch because it's shown like if her career was cut very short and if she was 26 at the time she would have already been on the older side for a national athelete). Either way, Jeon Do Yeon doesn't look 39 nor early 40s and it completely broke the inmersion for me. The cherry on top is the way FL acts and dresses, they decided to make her an old fashioned mother and it's also ridiculous when in the first episodes it's mentioned how she looks too young to have a daughter that age (when many of the other actresses playing mothers are around the same age than Jeon Do Yeon). The actress that plays Dan Ji's mother, Hwang Bo Ra, is 39 in real life yet no one mentioned in the drama she looked too young to have a daughter that age... lol
This was easily solved if they A) had taken a different actress or B) had changed the script for the FL to be closer to the actress' age. I don't care about the age gap of the actors, they simply don't look the age they're supposed to be, especially and mainly her. I clearly remember 2010 and where I was, so it's very immersion breaking that I'm to believe FL is anywhere in her 30s.

This also brings me to the fact the best friend also dresses and acts like an older woman. Why? Are they trying to mock working classes or something? (and it feels even more so when she's not dressing like that in the end when the time jump, now she's studying to be something more than a side dish shop assistant she doesn't have to dress like that? *eye roll*). I don't understand the purpose of this other than the clash that can visually produce between ML and FL (but then why the best friend too?). But this also brings me to another point, while I do think the main leads had physical chemistry and there were cute moments between them, they didn't fit together at all. They didn't seem to have similar views nor similar likes or anything, and his love for her was quite sudden, in the sense that it was shown as a sudden crush. I did like the progression of the relationship since it was more mature, but I couldn't get pass the fact they don't show anything they share that you would think makes them belong together (or am I to believe it's only because he knew her mother?).

When I started watching this drama I did have some expectations that being set around korean education there was going to be a bit more criticism of the education system, but I should have known better. While there are illustrations of problematic situations surrounding the education system there is not really any conversation about how problematic it is except for a couple of sentences in the last episode, where ML, I might add, suggests a division approach to levels that not only has been done before many times, but that it has been proven as many times to not work well, and this is coming from a teacher, the ML, that is supposed to be the best of the best.... hmmmm... ok. Also, except for the school teacher timidly mentioning the education system is a problem, the blame is generally put on the students, "Kids nowadays do ridiculous things. They consume highly caffeinated energy drinks and take pills. We even sent out several announcements asking them to cut down. [...] The educational system and parents' mindsets are to blame. [...] I'm a mere school teacher as well." So this guy thinks that parents have more power to change things than a "mere school teacher"...hmmm ok, no wonder things are like they are and Korea has such a level of teen suicide. System and those enforcing it bring teens to the brink of death and look puzzled at "kids nowadays doing ridiculous things".
This drama doesn't address the issues, only shows some of them but there isn't a real in-depth commentary on the problems of such system.

There are too many out of the blue subplots to acts as fillers. The thriller part of this show was predictable from the start, didn't add much in the sense of nuance for the main characters (ML already had that nuance from the past with Ji Dong Hee's sister's death) and it was solved in the most anticlimactic way possible. Similarly, the subplot they decided to bring for the last two episodes was dragged as well but solved quickly and without mention of any of the parts. Nam Hae Yi acts like a total brat and once the bio mother goes it's brushed under the rug like nothing happened.

Generally I didn't like much the FL here, on top of being portrayed as a quite stereotypical ajumma it irked me so much how she treated her brother, she was always kind of angry and having a go at him and only worrying about him (and anyone else for that matter) when she couldn't handle her own selfish anxiety. She's that type of person that seems she worries about others but only worries about the fact she's worried or will be worried, and so she pressures others to do as she wants so she doesn't suffer the anxiety... kind of toxic right?
I thought it was bad taste the way the "romance" between the FL's brother and best friend is portrayed, it's very out of nowhere, it's not developed, it doesn't grow and was extremely forced.

I nearly dropped this drama on the first half solely because of the mother's club, it wasn't entertaining, it was very annoying to watch even if I know many parents are probably like that. This also brings me to the fact that generally I found all characters to be very unidimensional, either they were good or they were bad. Yes, they try to tell me towards the end that Soo Ah's mother was actually not that bad, but I'm not sure how it can be excused that she pushed her daughter to the edge of death with her selfishness and obsession, same goes with Seon Jae's mother.

There were some cute moments and I did like the dynamics of friendships in the school (minus Soo Ah, that also her problems are never fully tackled either, they're just forgotten), especially Geon Hoo and Seon Jae, and I wished there had been more story to Dan Ji, since she seemed cool. I liked Hae Yi in the beginning but in the end I was very fed up with her.

Music was forgettable and annoying at times for me.

Overall I'm not sure to whom I would recommend this drama, since it's a mix of too many half-baked ideas and even the main one, the romance, is not very good. It's watchable if you have nothing else but otherwise no.
I generally don't tend to rewatch dramas but I certainly wouldn't this one.