"The Hundred Dresses"
Eventually, the bullies lose their favorite punching bag, because Wanda disappears from school. I had misremembered Wanda as dying, which is understandable, because there's not going to be any resolution for her anyway. Wanda's mother sends a note saying that they are moving far away to a big city where no one will make fun of her children ever again (does such a place exist? hmm, maybe she really does die). The cruel classmates learn that Wanda did have a hundred drawings of dresses, and they were exactly as she described them. They feel contrite, but eh, the victim has already moved away, so what can ya do?
Nobody understands an artist.
Estes, Eleanor. The Hundred Dresses. 1944. London: Sandpiper, 1974.
The Hundred Dresses