If you haven't yet read this article you MUST -- teacher, parent, grandparent, -- anyone who even talks to a child (or another person for that matter). This article is so enlightening. She did a master job of being proactive in terms of preventing bullying and dealing with the challenges some face when struggling with the roles others perceive for them.
One teacher being proactive when it comes to bullying.
As part of her inspired lessons she uses Tomie dePaola's Oliver Button Is a Sissy and Charlotte Zolotow's William's Doll.
If the students are somewhat older I would also suggest the Trading Places with Tank Talbott by Dori Hillestad Butler and also Butler's The Truth About Truman School, as well as, Carol Gorman's Games: A Tale of Two Bullies. Learn more about these books by visiting the authors' websites at www.kidswriter.com (or www.dorihbutler.com) and www.carolgorman.com. Both authors have developed successful sessions focusing on their books and the topic of bullying. Some interesting comments come about from these sessions. Makes some of us realize that this teacher's lessons on gender identity are so very important.