It's been so long that I don't remember, but wasn't "Sue Ellen Moves In" a flashback episode? Seems like I recall the teaser having Arthur reminding Buster how he thought Sue Ellen and her family were aliens when they first moved to this town, while the episode itself was a flashback to when the Armstrong family moved into Elwood City.

But it's kind of the same with Binky too: he was held back in Mr. Ratburn's class, right? Yet, IIRC, in some flashbacks, he's in Mr. Marco's second grade class with Arthur and the gang... and I know that when Arthur's had flashbacks of preschool, Binky was in their preschool class as well:

ARTHUR: Can't we shawe the cwayons? I want to colow a wainbow...
BINKY: Oh, okay! Here, I'll share this WHITE crayon with you! AHAHAHAHA!

Matter of fact, I think "The World of Tomorrow" is the only episode where we see Binky actually in Mr. Ratburn's class prior to being held back.