Madeline is a Reality WarperThe other 11 girls Took a Level in Dumbass while Madeline stayed at the hospital in the first TV special ("First we brush our bread, and then we...break our teeth?") because they are physical manifestations of Madeline's imagination, and didn't know how to function without her. Madeline also created Genevive to save her from drowning, and granted Genevive the relatively high intelligence she demonstrated during class. Pepito also sprang forth from Madeline's imagination because she wanted a cute Spanish boy to live next door.
In the movie-verse, Carly Simon attended the school at one point.The song "In Two Straight Lines" is sung from the perspective of one of the girls (who isn't Madeline). Moreover, she was born in 1945, while this movie has a Setting Update to the 1950s, which would make her old enough (just about) to be in (any, let alone this) school during that time.