It is found in the 1st floor of the Main Building Sections 1.
(But it's possible when you play the Hell mode)
In the original, defeating Machine Room Ghost in the Workshop and then out through the hole leading to the Home Economics Classroom, you get the key there to get out. It is used to open the Home Economics Classroom door. The key is discarded after that.
In the remake version, after defeating the Machine Room Ghost, it is thrown out of the the safe along with the Metal Amulet. After you get it out through Workshop door you give the key to Seong-ah, who claims she left her book in the Home Economics Room.
- In the remake, if you Choose the Seong-ah&Ji-hyun route in Hell [ Wang Real ] Mode, You can re-acquire this key at 1st floor in the New Building.
- If you use the key and enter the Home Economics Classroom, you can get the a Figure (Special Operations need) and Na-yeong as indicated in the hidden document.