For MMGM, I am recommending The Hundred Dresses by Eleanor Estes, with illustrations by Louis Slobodkin.
Here's the publisher's description:
Eleanor Estes’s The Hundred Dresses won a Newbery Honor in 1945 and has never been out of print since. At the heart of the story is Wanda Petronski, a Polish girl in a Connecticut school who is ridiculed by her classmates for wearing the same faded blue dress every day. Wanda claims she has one hundred dresses at home, but everyone knows she doesn’t and bullies her mercilessly. The class feels terrible when Wanda is pulled out of the school, but by that time it’s too late for apologies. Maddie, one of Wanda’s classmates, ultimately decides that she is "never going to stand by and say nothing again." This powerful, timeless story has been reissued with a new letter from the author’s daughter Helena Estes, and with the Caldecott artist Louis Slobodkin’s original artwork in beautifully restored color.
This book has been published since 1944, but it is just as current today. It has a timeless message about poverty and bullying. One of the main characters, Maddie, is friends with one of the bullies, but she never stands up for Wanda and feels awful about it later. Part of the reason the story's message is delivered so well is that it is not tainted by an incredibly happy ending. Wanda ends up moving away due to the bullying, and her classmates find out too late what the hundred dresses are and how much Wanda actually liked her classmates. However, the end is satisfying, and it allows the story to be much more powerful. Another great thing about this book is its length, which, although short, is perfectly long for the story. The illustrations also set a sort of happy mood, preventing the book from being too sad. With its amazing message (which helped it win a Newbery Honor in 1945), The Hundred Dresses is still a classic, and it is perfect for kids to read by themselves or for reading in class. (It may even help some kids become nicer!)