A gangster goes to a rural school to teach a class of five unique children.
Original Webtoon:
Naver Webtoon, Naver Series
81 Chapters + 2 Specials + Epilogue (Complete)
6 Volumes (Complete)

This series started out very strong. Every member of the small cast received quite a bit of characterization, the story moved at a good pace, and the author used the medium of manhwa to great effect (with respect to pacing, panels, etc.).
But after the first ~30 chapters, the pacing gets pretty thrown off (maybe due to the health issues mentioned in a few author's notes) and it often feels rushed, with jarring transitions between scenes, choppy action scenes due to not enough panels, etc.
The major plot thread also suffers from some bizarre issues. Nothing about that plotline makes sense if you assume that even semi-competent police exist. For instance, there were ~5 kidnapping attempts by the same guy on the same target, most of them in open daylight (and several with advance warning of the "I'll kidnap your son" variety), and yet both police and bodyguards prove absolutely useless (?!?). That threw my suspension of disbelief entirely out the window. The main antagonist also has absurd plot armor, somehow escaping unscathed every time (once, the MC beats him almost unconscious with the police close by, and he still escapes somehow, wtf). And even though the protagonist is depicted as pretty unbeatable in fights, in front of the villain he somehow becomes a punching bag each time (e.g. c49, c80).
Finally, the ending itself is abrupt and a bit of a letdown, managing to be even more rushed than the main storyline.
In conclusion, I wish I could've read the manhwa the first ~30 chapters promised it to be, rather than the one it turned out to be :/.