Spoilers ahead of you. Be warn!
Everything you see here are in fact, real information I gathered from a certain file source that was lingering on the internet. It really got my attention on it since the file was created by none other than the original White Day director, Yi Eun-Seok (Yes, he did publicly release it on the internet so it wasn't a leaked file). I will cover anything what was originally planned in all four of the buildings (draft plan as you could say) but I won't be covering everything here so I will split up into four parts (you know what it is). There might be new discoveries found along the way so do keep an eye out.
I believe the layout is what would happen if it wasn't start from scratch and the name (AKA Tragedy in Ghost School) continues to live up.
Main Building 1
There was no plan for having basement floor apparently.
It seems that the Janitor was supposed to drag In-Hyup and went to the Hallway instead? There's no indication where it happened at. It seems that upon entering the vent and "you know where the event happens", it just starts from there.
Home Economics Classroom and Machine Room (Workshop)
Apparently, Seong-Ah was supposed to walk towards the window and get surprised (self-explanatory) by the player. (Which explains in Tragedy in Ghost School trailer). What's interesting is that her role was supposed to given to someone else but got cancelled out. I wasn't able to determined the original name since the draft plan layout which was digitised (I'm assuming) isn't a high quality.
Instead of using the "Metal Amulet" to get the fire shutter door near the office open, you would use the "Wood Amulet" instead.
In Classroom 2-6, you would find the "Wire Cutters" there.
Classroom 2-5 to Classroom 2-1
Registering Room (Student Records Office) and Main Office
You would find Ji-Hyeon searching the desk in the Main Office instead (Though, it doesn't match up with the trailer).
Door to Main Building 2
You would find So-Young's Locket Necklace in front of the door.
You would find "Principal's Office" written in blood instead of the bloody handprint.
Gym Storage Room
Master Key would be located in the cabinet instead.
You would find "Earth" (Not known if it is the seal or the amulet) there.