I'm only 3 episodes in, but...
So far it seems like a pale imitation of All of Us Are Dead with a dash of Happiness. The action scenes are ok but trying to watch them just before bedtime was not my best move ever. The students don't seem too bright -- the non-action parts of the show could have used some redesign.
Space starfish just don't seem very credible as zombie stand-ins.
Are the alien spheres hanging overhead for a year without doing anything while people just get used to them intended as an allegory for the long-term threat of attack by their neighbor to the north? #swordofdamocles Then the spheres landing would stand for a invader's artillery and missiles.
The platoon leader seems to have magic unlimited ammo.
They need depleted uranium to kill the starfish but they don't have enough of that ammo for their rifles? I'm surprised they have any -- I thought depleted uranium was only used in big tank guns to deal with other tanks. And have they tried silver bullets or wooden stakes?

My thoughts on the first six episodes:

First, a disclaimer for anyone who is sensitive to gore. It might be best to avoid this one. It's certainly the most gory KDrama I've seen. I'm not sure that it's the most gory out there because there are a few I've avoided knowing they were gory.

The positives...once things get going, it gets pretty intense. That kept me hooked from episode 2 or so. Most of the acting is decent. The concept, in my opinion, is pretty good.

The negatives: most of the negatives center around the kids. Quite frankly, it is unbelievable (at least to me) that so many of them are in denial for so long. And they are pretty stupid in their state of denial. Of course it makes sense that civilians would have been moved to shelters in the midst of this level of emergency, but several episodes in many of them are clamoring to go home. Of course it makes sense that if the military has conscripted you in the midst of a worldwide emergency, you are going to need to get with the training, or go AWOL. Instead, many of the girls and some of the boys turn into crybabies.

Although, it denies credibility that the military, no matter how desperate they are for manpower, would let some of these kids out on the battlefield, even as cannon fodder. The ones I am referring to are a danger not only to themselves but also their fellow soldiers. If something like this had happened in real life, they would likely have been the ones digging latrines.

This six episodes of Duty after school are really entertaining, specially since the second half of Episode 2.
Some of the teenagers are annoying, but I find the series portrays most of them in quite a realistic way. They are just kids in a terrible situation.

I have enjoyed watching Shin Hyun Soo in this Platoon Leader role. Very different from the other dramas where I had seen him (Age of Youth, 12 Nights).

(I can't compare this drama to All of us are dead because I have not watched that yet).

My thoughts. Episode 3 of 'All Of Us Are Dead' had viewers bawling their eyes out, so much so that crying Youtubers became a meme, like with episode 6 of 'Squid Game'). I can't imagine shedding tears over the cast in this drama. As of episode 2 they're little more than cardboard cutout characters. At this point I'd class this series as 'Something to watch because nothing else is on', a category I usually reserve for mid-level C-dramas.

Don’t look at the plot. It’s as holey as it can be. On emergency situations they chose to trained boys and GIRLS to go into the war while their teachers as adults just sit back and didn’t bother to hold a gun?

If you want to look some good suspense with a little high schoolers comedy and okay with quite a lot of stupidity you can watch this.

it is partly annoying and partly necessary. what with war going on in Ukraine, people forget that death is real. but im less interested in the survival of the students that the evolution of the alien specie. were they observing and learning for a year? what is with primitive lifeform and strategic planning? either it is biological ignorance or an environmental statement: humans might not be the dominating specie one day. what if nature catches up and surpasses us.
it does need some scientific reasoning as to how can a squid brain evolve like that. can a superior mind live in a body / form we normally don´t connect to high intelligence? is it more mimicking or is it real learning? the emphasis on college entrance exams, studying without knowing why, versus learning as an act of survival. Interesting. so far, I see very little of the same focus in students: are they learning anything? are THEY evolving as fast as the spheres? It is not just an alien invasion thing, it is a race in evolution. Do humans justify their claim for superiority or are we clueless losers, dependant of technology, weapons, do we know what we are doing or are we just killing time here? what is the end goal of life as such?

in a way I want the spheres to win cause if humans dont evolve, what is the point to our existence?

I watched the whole thing. I actually enjoyed it … mostly-ish. Until the last episode. I’ll not spoil it but … what df were you thinking writers ???

After spoiling myself and read all of the Part 2's review available online, i decided to limit myself till the first 6 episodes. Shin Hyun Soo is perfectly in character and I dont want his platoon leader's sacrifice and the memories he made with the students go to waste.