Mr. Larson is a one-time character that only appeared in the episode "Nursing a Grudge". He is a teacher in Tommy and his friends' school and his subject is science.
While having a science test, [[Gus Turner|Gus] ]tried to find a way to skip it, so he put a wig full of lice on his head, which allowed Mr. Larson to tell him to go to the school nurse and thus, getting rid of him. Excited over this, he gave the pupils candy, sent them to recess and cancelled the test altogether.
After Gus gets his brain swapped with a pigeon, the pigeon in Gus' body shows remarkable improvement at doing tests to Mr. Larson's impressment, and hd also thanks him for the eggs he gave him.