I've seen many people saying that the drama was to repetitive , I don't disagree with that, but it my opinion it wasn't a bad thing, it doesn't make the drama tiring , instead I think it creates a whole scenario of loop that will create the creepy atmosphere to the story.

I was really enjoying the series, but the ending disappointed me a little bit, I thought the ending was too short for the whole scenario created, which lead to things being left unexplained, some of those things took me a while to understand by myself and others I haven't figured it out yet. Another thing that confused me was the cut-scene at the very end after the credits, I couldn't quite understand if it was after the end of the series or if it was during it. But I think that if the author had made the ending a little longer and explained things better the drama would have a lot more positive reviews.

Also they could have chosen better actors, I mean, they weren't bad, but sometimes I wasn't feeling the creepy vibe I was supposed to.

Aside from that, I think drama was good.
The atmosphere created was amazing, it was creepy and uncomfortable, the music the scenario and the effects, all contributed for that.

I also loved the idea of the story, a creepy school where unusual things are happening.

One thing that made me really feel like it was creepy and thrilling were the moments when the kids going to make a decision and I would start nagging at them saying it was the wrong decision.

I did liked the drama, except for the ending, but I would recommend people to watch it because it's interesting and since it's short, it doesn't take that much time to watch.

I'm thinking about rewatching the series when I have time so that maybe I'll be able to understand the ending a little better.