What do dragons symbolize in the novel, Ms. Bixby’s Last Day?

Dragons symbolize the challenges of life. They come in all shapes, sizes, and forms, and they can appear in the form of angry older sisters or jerks like George Nelson, while others cannot be seen on the surface. Such a dragon is cancer. While the boys are able to come to make Ms. Bixby feel better at the hospital, they cannot defeat her dragon.