This is one of those films that you really need to watch at least twice to really get the hang of what's going on.
First time I watched it I couldn't get my head round what was going on - and I was actually more shocked by the brutality of the male teacher Mr Oh.
I don't like seeing violence against women or girls, even in movies, and this was just disturbing... but then I suppose that was what the director was wanting to show. Didn't make it easy to watch though.
As for the horror content - it's not a scary film in the slightest.
The ending might be seen as overly sentimental, but I found it very moving.
I don't have many Korean films (I prefer Japanese), but the ones I hava are good, and this is no exception.