Danny Antonucci said that he'd never show an adult in the series. Also, here's a little trivia, did you ever realize that there are no CD players, MP3 players, or DVD players in any episode? That's because all the episodes, as seen in one of the episodes of season five, shows that all of the Ed's adventures are just Eddy's memories of childhood, making the time frame of the show in the 1930's / 40's. It would also explain why the students use typewriters instead of computers.
At least, that's what Wikipedia said ;)