I kept thinking, at the end, that the school district was going to have a really big expense line in facilities after this. Fortunately South Korea has a lot of unused schools where people have moved away from smaller towns, so I imagine some can be re-purposed.
Seriously, I am really sorry this is as good as it is. I hate the recent increase in violence in K-dramas, and this was violent in the extreme. But the story is great, the acting is awesome, and the characters are extremely well developed. I ended up watching some sections with one hand on the 10-seconds-forward button, because really, you can watch people being pummeled only so long.
And oddly, the ending did not disappoint, although I could have done without the coda after the credits at the end. All in all, a surprisingly excellent drama if you have the stomach for violence, so be warned. Once you start, you'll find it hard to turn off.