Psychedelia(2002)

Psychedelia

High school senior Gwon Kang-Han lives a fairly ordinary life with two troublesome parents. He thinks he's been playing too many RPGs when he has a strange dream in a fantastical setting, but that isn't the end of the odd experiences he will encounter, as he discovers the next day at school: in fact, his adventures seem to be just beginning.

Source: Baka-Updates