How did the letter written by Wanda’s father affect the children in Room Thirteen?

When Miss Mason reads the letter to her class, Maddie feels awful, even though she knows that she never took part in the teasing, but that she hadn't done anything to stop it either. Peggy, however, seems oblivious to the fact that their teasing was hurtful. After school, Peggy and Maddie both have the idea to go to Wanda's home to talk to her.