The anime is following the light novel. The manga is too but it's cutting out all the content involving the classmates so I don't know who they're going to give us important info come that part of the story.
Oh so in the light novel, they will tell this story and the classmates' story in parallel?
Yeah. It's done on purpose and slowly reveals the entire truth of the situation. I like the manga because Kumoko amuses me, but seeing how things are supposed to play out in the anime and light novel is very interesting and keeps me on my toes though the classmates aren't a favorite of mine. You'll get an idea once you've looked at both.
Is this manga based on web novel or light novel? I want to read the one consistent with this manga.