Principal Sezno, Helen Klench, and Stuart Proszackian are voiced by Saturday Night Live cast members Kenan Thompson, Will Forte, and Cheri Oteri (Kenan Thompson has been on the show since 2003 [and, as of the 2014-2015 season, is still on the show], Will Forte was a cast member from 2002 to 2010, and Cheri Oteri was a cast member from 1995 to 2000).
Happy the Janitor and his English translator are voiced by Tom Kenny (a Mr. Show cast member and common voice actor on a lot of 1990s and current kids shows)
Miracle is voiced by Kristin Chenoweth from another short-lived show: Pushing Daisies.
Missing Episode: "Math Lab," which was originally supposed to air after "Back in Time," but due to its content (and the fact that the show wasn't bringing in the big ratings that FOX wanted it to), the show was cancelled after the fourth episode. "Math Lab" — and the other eight episodes that were supposed to air on primetime Sunday after The Simpsons — ended up airing on late Saturday night when FOX was trying to fill the void left behind when MADtv was cancelled.
Also, when the show aired on Comedy Central, the pilot episode and "High School Confidential" never aired.