Poseidon Elementary School has a rather unique shape with the top being red in color and the bottom being black in color. It has a set of double doors and five windows on it. The school's name is written on the top section in orange.
Role in episode
Mr. Krabs and Plankton were both laughing stocks at this school, primarily to class bullies led by Billy. Mr. Krabs was called Rag Boy because he actually wore cloths and rags, and Plankton was made fun of for being diminutive yet intelligent and because he was Mr. 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 Mr. Krabs and Plankton, for the same shallow reasons as the children of Billy's gang. A teacher could be seen, but he was not teaching any lessons nor stopping other students from teasing Mr. 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.