Of the same ilk as "All of Us Are Dead" (Korean high-school zombie disaster mini-series).
Duty After School is adapted from a shorter Korean webtoon. Noticeably slower pace of action compared to other similar thriller series. A LOT of Part 1 is spent on the angsty emotions of teenagers being unknowingly drafted into a war against aliens. Lots of scenes with the usual social/political jibes at the Korean education system, bullying, yada yada you know the drill by now.
When there is any action, it's spent on the kids running around like headliess flies, screaming and panicking. Period of calm are denoted by even more angsty teenage scenes with angsty emotions. Did I forget to mention almost all the male kids are angsty and behave like 12 yr olds?
The age and behavior doesn't make much sense ... since they are all supposed to be 17-18 (final year of highschool). Several countries like Israel, Singapore and yes, even S. Korea, start their mandatory conscription draft at age 18. And they definitely don't behave as immaturely as the kids here do.
But hey if you're like me, with nothing better to do and easily entertained watching disaster films, all while complaining about the infinite plotholes and even more infinite ammo, this is the perfect way to waste a Sunday.