I've recently started watching the show again: it's brilliant. A brave mix of surrealism and slapstick that is not far removed from the most far-out Looney Tunes. One of the stranger parts of the show is the lack of parents or other kids in the series. The entire run consisted of only the core 12 characters, with one justified exception in the grand finale. The final season even shifted focus into the kids' school, and yet, no teachers, principal or other kids are ever seen. What gives?
I believe this is just a severe case of conservation of detail: the surrounding people were not important, just the exploits of the Eds and the other kids. This forced the show to really flesh out the characters they had created rather than come up with new ones to fulfill certain roles. The result? Excellent characters, all with unique traits and style. The later seasons turned the focus of Ed, Edd n Eddy away from the plots and began introducing more purely character-driven interactions, including an odd Jonny-Nazz storyline in the 5th season. Who knew you could get so much out of so little? It's the concept of minimalism--the sheer lack of options available made the slightest change seem far more significant.