God damn, I've been reading about this for 4 hours. It's 4am now. I need to go to sleep.
Here's the 299/300 chapter spoilers.
Here's chapter 299/300.
Basically, it says that Kafuka was dead all along. She died in a car accident. She was an organ donor and here organs got into the 17 girls, which attempted suicide once. There's also some stuff about Showa era ghosts who also attempted suicide living in them and giving them a will to live or something like that. When one girl wears Kafuka's hairpin, she acts like her and the others who also got Kafuka's organs see her as Kafuka.
And this was all planned since the beginning.
Her name (Anne of Green Gables = Akage no An/赤毛のアン (Akagi An)) was in the very first chapter.
A tail besides her name, implying she's on Abiru's body.
Go watch the OPs/EDs, too. It will blow your mind.
Some more stuff on /a/ if you want to further blow your mind.
AND NO ONE NOTICED. God dammit, Kumeta. You glorious bastard.