I haven't read the webtoon so this is based only on the series.
This was an interesting take on the apocalytpic genre woven into the dynamics of being a high schooler. The idea is quite creative and while the storyline could have been much more intricate and the season longer, it's still a good watch if you want something quick and adrenaline rushing. I would have definitely liked more episodes focusing on the backgrounds of the characters in depth, and the bonds between them and their superiors. For a long time, I couldn't memorise their names either because there was not enough spotlight on a few characters at one time...

There were some annoying and frustrating characters such as the cowardly girls who wanted to leave everything and run away. The constant whining, scream-crying almost made me drop the episode or skip the scene, especially when the dialogue repeats itself at least twice in a single episode. I almost felt like slapping them so i'm surprised at how and why the opposite characters were soooo good at holding back and not lashing out at them in times of emergencies (it would have been understandable if there were more clashes)

P.S. it's better to assume that this is a solo work with an open ending instead of watching season 2. You're better off without it :-)