Asian horror movie featured on Sundance channel?

[Katy]

Awhile back me and my mom were looking for movies to watch and there was a horror film festival on... We clicked on this one movie, it was about this school reunion... anyways there's this one kid who's deformed and constantly picked on for it and lives in the basement... the one kid gets revenge on all the students.. but in reality that's what the one girl that continued to live with her teacher said. Does anybody know this movie