Well, I say everybody, but as this episode begins three days after Yuri's battle to end both the reign of the shadow monsters and her own inner demons, we find that all but a handful of characters have disappeared and moved on already - I guess you know you're only a side character when you don't even have your back story or resolution of that storied aired on-screen, so we might never know what ailed TK.

To handle these last goodbyes for the final five members, they hold a faux graduation ceremony, complete with a suitable cute anthem devised by Kanade and the handing out of certificates, before Naoi, Yuri and Hinata alldo their part and disappear from the scene, leaving just Kanade and Otonashi alone.
What happens next is initially unsurprising, as Otonashi suggest that he and Kanade stay behind to help anyone else who might stumble into a sequel... err, I mean stumble into this world - A decent enough idea, which Otonashi ruins by professing his love for Kanade. It isn't that this feeling is reciprocal, but rather that Kanade's admission of such would be the cause of her disappearance; something which happens eventually anyway, although not before a revelation that makes absolutely no sense unless we're also going to throw a little Doctor Who-esque time travel into the mix. Either that or Otonashi, spent some of his life doubling for the Tin Man in The Wizard of Oz.
Funnily enough, this final scene exhibits a problem that recurred throughout Angel Beats - Its plot often resembled Swiss cheese... the kind of Swiss cheese with holes you could drive a bus through. While some series would get destroyed by me for exhibiting such disregard for continuity and common sense, this show managed to avoid that largely on account of being one Hell of a lot of fun to watch. However daft its premise and sub-stories became, Angel Beats never stopped being entertaining, frequently progressing thanks to its ability to mix drama, action and humour with gay abandon; when you're laughing or simply drawn in by the on-screen happenings, it's hard to start dissecting problematic plot points.
So, Angel Beats is no classic, and if we're honest it could have been so, so much more. But hey, as anime goes I think it's perfectly alright for us to accept and show a little love for its flawed existence, so that it too can graduate and move to its next life - Who knows, maybe it can come back as something that is greater than the sum of its parts, rather than simply being a handful of admittedly entertaining parts whose sum was a rather slapdash affair that didn't quite know how to combine them all.