Fountain Guns are water fountains that function as cannons. They are found in the hallways of Hendry Middle School. Whenever an intruder is present at Hendry, it will shoot intruders down.
During the Gallagher Elementary School's assault on Hendry Middle School; Pauley, Jesse and Kimmy raced inside the school halls and were attacked by the fountains. Both Jesse and Pauley were shot down, and Kimmy managed to escape.
They appear to be based off of the Death Star's Turbolaser cannons of Star Wars.