Walter may be almost as old as Jem, but he's still in Scout's class at school. (In other words, an almost-ten-year-old is sitting in with the first graders. Of course, most of them are repeating, too, so he probably doesn't feel too out of place.)
On the first day of first grade he lacks both shoes and a lunch, but it's clear he's a step up from Burris Ewell: he has clean clothes. Scout tries to explain Walter's lack of a lunch and refusal of a loan to the teacher since Walter himself can't or won't, but Walter's situation—too poor to pay back a quarter—is just incomprehensible to Miss Caroline.
Of course, Scout's own ability to understand is exceeded when Walter pours molasses all over his lunch at the Finches. Thankfully, Cal's there to tell her that just because someone's different doesn't mean she gets to judge them. Of course, she gets a lesson in the other direction when she wants to hang out with Walter but Aunt Alexandra squashes that idea because the Cunninghams, in her eyes, are trash. Walter, as a Cunningham at Scout's age level, serves as her gateway into the complex world of white class relations in Maycomb.