spoiler...
I havent fully read the novel but i do know some spoilers. Because of that, what I say here may not be fully accurate.
From what i know there's like a pre timeskip (when shiro first became a spider) and post timeskip. Her other classmates only show up post timeskip, and it's important to note that there's a time gap from when shiro reincarnated and when the other classmates reincarnated (like the vampire girl. Shiro was already an experienced person in this world while her other classmates had just started being born)
I've read some parts of the novel and i think the manga follows it pretty closely (until where I read at least). So no, they're not going by the novel. I think they did this to compress the plot because this does have a bit of a lengthy story, but I think it just confused anime watchers and manga readers alike. Plus, the appeal of the early parts of this story was watching shiro get stronger as time went on, while also wondering if there was more to this world than what we see through shiro.
:(
Also it's because Shiro was reincarnated as a spider while most of the other classmates were reincarnated as babies. So considering the growth rate, we get the already grown up version of the classmates cause their baby experience would have been boring (they're all boring to me Imo. Shiro's is more fun)
While shiro grows up faster as a spider.
So by the time the babies grow up into young adults or ..teens?
Shiro has already had so much growth! And vampire baby too because of all the ruthless training she goes through as a literal baby
When I watched the anime, I was hella fucking confused. Why the actual fuck did they ruin the plot by deviating from what actually happened in the manga. Are they skipping shit and going off of the novel? I was so confused after seeing her classmates in the anime that I dropped it because it was ruined for me.