Jack Black makes a jarring and unexpected appearance in the Season One episode "Investigative Journalism" as a new student named Buddy. But the incongruity of seeing a high-profile star in a (relatively obscure) sitcom doesn't go unmentioned. In fact, it's pretty much the entire point of Black's cameo.
Buddy turns out to be obnoxious, desperate to be liked and completely unable to gel with the study group, and the scenario self-consciously mocks the way guest stars are contrived into sitcoms where they clearly don’t belong. Buddy's tendency to break into song, his broad, sexual comedy and his position as the "chubby, agile" group member also parodies the Tenacious D singer's over-the-top comic style.
Owen Wilson pops up at the end as the celebrity member of a rival study group in order to complete the gag.