This drama is actually really good...
First off, I like how the zombies in this drama aren't those typical green, slow, stereotyped zombies that are seen in mostly cartoons and western shows. These zombies are different - they're a lot faster, more realistic, gorier, and has (normal, not green, which is unrealistic by the way), skin.
I like on how zombies in this drama are attracted to one, specific thing and have a distinct, foolproof weakness - which is sound. Generic zombies are quite different as to what they do is just walk around slowly, wanting to eat brains. They were smart in the way of thinking, because everything has a weakness - and they had to find a way to combat it.
Basically what I'm saying, is that they have improved and differentiated the types of zombies a lot, which is quite creative and something that makes this drama different from other zombie films.
The characters are well developed, and each have their own differing personalities. There are also a lot of characters, which isn't a bad thing, although may cause slight confusion when refering to certain characters. Su - hyeok is the athletic one, helping to keep zombies off his classmates. Nam - ra is the smart one - known as the class president, she is able to use her smarts to make techniques on how to escape the school, (with zombies everywhere) for example. There are also a lot more characters with their personalities.
One thing though, that comes with lots of traits and qualities, is that there is lots of arguments and contrast with ideas and the "what to do next," factor. This can be seen when Nayeon - (who can be seen as a minor antagonist), has conflict with a character named Gyeong su, and the argument ends with Nayeon (deliberately) turning Gyeong su into a zombie. This is an important part of the plot, as there is a sense of betrayal when she does this. They were supposed to help and keep each other alive, not turning on each other when one may have had a different opinion or maybe made a slight mistake. This is where everything gets exciting, as the climax of the plot slowly continues to reach upwards. In other words - its realistic, its exciting.
One thing that I will say may have put down a point for me, is that they may have "stretched" the plot out a bit. By this I mean is that they put a whole zombie apocalypse throughout 12 1-hour episodes, which sure is realistic but in the long run, might get viewers uninterested. There is no additional plot - just a zombie apocalypse for a full on 12 hours.
Another thing that I may add is that they repeated scenes and scenarios for a bit, where all they would do is hide in a classroom, form a plan, get out of the classroom, and that would go on for a long time. Yes, I understand that it might've been difficult to add in something else, but when you really analyse it that is really the only thing they really do.
its good though defienetly watch 10/10