Alexandra is a strong believer in Southern tradition; one of the traditions of the Old South she continues to uphold with great zeal is that of social class distinction. Put simply, Alexandra is a snob. She is enormously proud of her family heritage as a member of the Finch family, one of the oldest and most respected in Maycomb County. As a result, she looks down upon any family that is not as old and accomplished (or as financially comfortable) as her own. It is from this frame of reference that Alexandra explains to Scout why she cannot be friends with Walter Cunningham, a classmate who comes from a poor but proud family:
The thing is, you can scrub Walter Cunningham till he shines, you can put him in shoes and a new suit, but he'll never be like Jem. Besides, there's a drinking streak in that family a mile wide. Finch women aren't interested in that sort of people.
When Scout persists, Alexandra speaks as plainly as she can, showing her ignorance and arrogance about Walter, a respectful little boy who cannot help the circumstances of his birth:
Because--he--is--trash, that's why you can't play with him. I'll not have you around him, picking up his habits and learning Lord-knows-what.
Her Aunt's contempt and cruelty reduce Scout to furious tears. Jem leads her sobbing from the room.