The Bookwalker: Thief of Tales

In book 3, after the special item goes missing proceed into the main hall.

You need to have pretended to be a janitor the entire first part so you will still have the mop bucket.

Some students here will ask you to help pull a prank on a teacher.

Tell the teacher his classroom is in the basement.
Go downstairs and talk to the pranksters and the door. You will need either a crow bar or the key to open the door.
Go back upstairs and enter the middle dorm room on the left.
Talk to the student, splash him with mop bucket, follow dialog and ask about the secret cache.
Go upstairs and enter the left office.
Ask the "old headmaster" where to get a drink and he will give you the basement key.

Go back downstairs and use the key to enter the basement room and fight three enemies.
You have to kill the strongest enemy first to prevent the others from reviving.