>suddenly serious melodramatic subplot
It set the tone right off the bat. You're confusing character design with tone.
It's always had a serious undertone to it. Part of the comedy of the show is being whimsical 90% of the time and then throwing the characters into some pretty dramatic situations.
You know, kind of how the first fucking season ended. Or how the first season's antagonist got introduced. Etc, etc, etc.
Here have this thing that was literally in the first episode
>OP is a faggot
What else is new
It's not even that. The concept of a super octopus threatening to destroy the Earth and the fact that the government enlisted a bunch of high school students to kill him was amusing, but then they threw in some sappy story about how those kids are the lowest ranking students and it sucks to be in that class because everyone looks down on them.
The government didn't choose the kids.
He just said "Wanna know what? I'll give these little assholes a year to do it before I destroy the Earth. You can teach them whatever you want, but it's gotta be these guys, not anyone else."
>high school students
Congratulations, you didn't even watch the first episode
Or read a synopsis
>some sappy story about how those kids are the lowest ranking students and it sucks to be in that class because everyone looks down on them.
This is the main reason why it's so popular with nip kids. It's their version of wish fulfillment that reincarnation/mmorpg stories do for older otaku. In Japan, which school you go to and your grades will decide where you go for college, which decides your chances of success in your future desired career. It's a lot less lax than it is in the west.
Imagine some kid being grinded through this system discovering AssClass. The octopus teacher goes out of his way to help you reach your fullest academic potential by going out of his way for you and giving you effective individualized tutoring. To the point where the beaten down underdog class can academically compete with the best.