He is frustrated with teaching Huey, who is not only far smarter than an average 4th grader, but also frequently accused Petto of brainwashing his students on behalf of higher powers. He is also annoyed by Caesar's class clown antics.
Petto also appears in the show, but for some reason he instead teaches a 2nd grade class with Riley in it.
In "The S-Word", he caused a lot of controversy when he called Riley a "nigga", with Riley and his Granddad threatening to sue over the incident. Petto claimed to be confused as to how offensive it could be taken, given Riley's constant use of the word, and the hip hop songs apparently advocating its usage. The incident eventually grew into a political debate, between the left-wing and the right-wing. Throughout, he seemed regretful and apologetic for using it, admitting his own confusion over the word.