Poseidon Elementary School is a local elementary school where Mr. Krabs and Plankton attended as juveniles. They were both laughing stocks at this school; Krabs was called Rag Boy because he actually wore cloths and rags, and Plankton was made fun of because he was Krabs' friend. Mr. Krabs and Plankton viewed their experience at the school negatively. The kids of this school go to a local burger joint called Stinky Burgers, owned by the man Stinky. Stinky liked all of his kids, except for Krabs and Plankton, for the same reasons as the children. This specific school has only been seen in "Friend or Foe." A teacher could be seen in "Friend or Foe", but he was not teaching any lessons nor stopping other students from teasing Krabs or Plankton.
It is named after the ship in the 1972 movie The Poseidon Adventure, which is a movie about a ship that flips upside-down. This is further evidence by the fact that the school resembles a capsized boat.
It could also be named after Poseidon, the Greek god of the sea.