accidentally stepped onto a portal where he ended up at the human world (Which was at Tokyo Japan) in Chapter 1. There, he met a boy and a girl named Hisao and Ayaka. Sackboy left the human world in chapter 3.
Sackboy's World and the Human World connects together to make a portal to both worlds.
Why Sackboy ended up at Japan because the manga originally released at Japan.
The school is a place in the Human World that Hisao, Ayaka, and other humans attend. It appeared in chapter one of volume 1, Going to the Human World. There is a room with lockers there. Hisao picked up Sackboy's pod computer and putted it in his bag, worried that Sackboy had to find it. When Hisao went into the classroom, Sackboy was there. While escaping from Sackboy, Hisao gets tired of running. Sackboy asked Hisao to give is pod computer back. Hisao gives Sackboy the controller and he needs to find the rest of his pod parts which where fallen into the human world.
The house is where Hisao took Sackboy to. There he introduces his sister Ayaka and his cat named Sumi. Note: This is not finished yet.