What are the "specs" of the afterlife world?
So for this bunch of questions I am going to use spoiler blocks as they are explained in the anime.

Limitations to school grounds:


   If I recall correctly, Yuri mentioned something like "behind the school boundaries there is a large and wide area of nothing but grass" and that she tried to escape, but was not able to.


Based on any actual place:


   In general, buildings are partly based on real buildings or generic. I would guess this school is a rag rug of multiple buildings mixed, excluding the underground facility of course ;)


Why not everyone / other afterlife worlds:


   Well, everyone who dies with regrets seems to go to a certain place, where they can get over these regrets and live a new, untainted life. Also, most of the students are mere "filler material" / "background noise" to create a perfect environment. They were created by the "Programmer". This person wanted to create a perfect world for students to clear their regrets. Thus the school is only for students. This means a part of Japanese highschool students get to this special place created for them. For other people dying with regrets, there "must" be a different place to go.