Manga/manhwa about sudden apocalypse which had creatures coming out of spheres. The main characters are high school soldiers. Everyone dies in the end

Question

I read this a long time ago (2019/2020) so I might be getting a few different manga/manhwa mixed together.

I believe it was a sudden apocalypse where there were these purple(?) spheres that randomly appeared across the world, from really small to massive.

I believe creatures came out of them - so the main group of characters had to train in the army to fight against them. I think they were high school students. They used modern weapons, like semi automatic guns etc. Definitely no magic.

The manga/manhwa was quite dark, with themes of bullying and death. I read it all and I know in the end everyone dies in the main group apart from one annoying guy.

I believe the manga was black and white, but I must have seen the cover at some point to believe the spheres are purple.

Answer 1

Could this be Duty After School (AKA Afterschool War Activities, AKA Afterschool Military Activity, AKA 방과 후 전쟁활동)?

Class 3-2 of Sungdong High School was just like any other class: a mismatched group of individuals preoccupied with their crushes, upcoming university exams, and other ordinary day-to-day worries. Yet, when strange purple orbs rain down from the sky one day, the students find their lives changing beyond the limits of imagination.

Capable of sporadic and sudden destruction, these spheres quickly send the world into a panic. With the South Korean military forces stretched thin, the government hastily enlists those in secondary education and above into the army. Now drafted into military training, Class 3-2 must abruptly set down their pens and pick up rifles instead.

Promised that they will encounter little danger—and that bonus university points await them post-service—the reluctant students are coaxed into accepting their new duty. Marching onward with an apathetic and easygoing attitude, the class begins their military service, unaware of the grim reality that will soon erode their innocent, carefree spirits.

Note that this isn't an exclusively black-and-white manhwa. The spheres (and various other objects) are depicted in colour for emphasis.