The Hundred Dresses

TL;DR. It's not a picture book. It's a chapter book with illustrations here and there. Basically, there's a poor girl who always wears the same dress to school she's Polish, with a weird last name, and kids make fun of her and she claims to have a hundred dresses at home and 60 pairs of shoes, but that just makes the kids make fun of her more. Then the main character girl feels bad about it and wants to apologize, but the other girl moves away so she never really gets the chance. The class sends her a letter and she responds and thanks some of the girls specifically, including the main character girl. That's it. It's too long.