Mr. Rustworthy is the music teacher at Jerome Horwitz Elementary School. He appears in 2 books so far. In the 5th Captain Underpants book, he witnesses the children having a food fight and the football team destroying the teacher's lounge. He is a nice teacher who likes children, especially George and Harold (both facts are stark contrast from most of the other staff), but not food fights. His name is a pun to the word "mistrustworthy."
He also appeared in Captain Underpants: The First Epic Movie. He does not do anything, being a minor character.