A grille-covered drain that was the terminus of a large-diameter grey plastic pipe outside the staff room.

Completely innocuous and unlikely to cause harm or even dirty your shoes, but having been given the nickname, being shoved into that deadly zone by an opportunistic fellow pupil would earn the unsuspecting victim many hours of bewildering taunting for having breached the "Poo Pot".

Possibly speculated to be the source of the "fleas" that everyone was so terrified of inheriting by any kind of glancing contact with the wrong sort of person (girls).