White Day: A Labyrinth Named School is an extremely underground (well, probably not so much anymore) Korean game that was never developed for the West. You'll have to download a modded version in order to play in English.
Because of the game's current state (read: abandoned), it's free to download straight from the developer, meaning no pirating! Just be sure to grab the English version unless you want to change your PC to Korean.
White Day: A Labyrinth Named School was deemed one of the scariest games of all time--or would have been, had it been released in the West. Some of the mods have actually toned the game down due to complaints that it was too scary to complete.
You play a young Korean student who transfers to a new school, and immediately falls in love with a fellow classmate who happens to lose her diary. As White Day (one of Korea's romantic holidays) is upon him, the protagonist sneaks in to the school at night to plant her diary and chocolates in her locker. Little does he know that the school is about to turn into a never-ending horror house.
White Day: A Labyrinth Named School is only available through download on the official developer site or through a mod community. Download with caution!