How does the author address the theme of appearances in the novel, Jason and Marceline?

In the novel, Spinelli addresses the theme of judging people on the basis of appearance alone. Jewel Fiorito, for example, develops physically so much during the summer that the boys do not recognize her when she returns to the ninth grade. They decide that since she is so well developed, she must be sexually active. Jewel become the object of undeserved lies and slander.