Delete the contents of files inside here: "C:\Program Files (x86)\Steam\steamapps\common\Summer Nightmare\game\saves" and here "C:\%username%\AppData\Roaming\RenPy\SummerNightmare-1503592906" (2nd directory may not exist if not don't worry about it, and just clear the first one).

I believe the reasoning for the achievement's not working is due to the game being shipped with a save that already has a persistent save file with all the achievements as unlocked on it.

START
Go to music room.
Go to the library to find some clues.
Go to HR teacher.
[SAVE 1]
School office

[LOAD 1]
School Cafeteria
This is none of my business.
Open the big pot.
Enter the room and lock the door.
Press.