I've been stuck in the part where you go to the library during the Halloween night for hours

[NoomTsap]

The library door is locked and i have no idea how to open it.. any help or small tips?

[Marble]

(Going by memory, may not be 100% accurate)
Go into the bathrooms, try and exit the top and they will be locked.
Then go back to the janitor, he's stood near a door that was previously locked, this takes you to the next "dungeon" to make your way around to the library from a floor above.