I teach 3rd grade single-gender...all girls! I have definitely had some of the same issues. Here are a couple of things that have seemed to "stick" with my girls.
*The movie another poster mentioned: Charissa Stands Strong, from American Girl. There are also tons of resources related to the movie on the AG website. We watched the movie and used some of the site resources to have small group/class discussions. The movie has many parallels to The Hundred Dresses, so it's a great way for the girls to make connections.
*During a class meeting I take a piece of plain white paper and cut out a heart shape. We imagine that this new/clean heart represents the heart of a girl who has never been bullied/made fun of, etc. Then I have the girls say things outloud that they have heard (or even said themselves) that are hurtful. As they say them I crumple the heart, tear it a little, basically mess it up. Then I ask them if it can ever be the same. We "try" as a class to make it look like it did before. They say apologies, offer "gifts" etc., while I try to flatten the heart and make it look nice and clean again. To wrap up, we talk about how once those hurtful words are said the heart can never be truly like it was before. Apologies can help...but once we use hurtful words they cannot be truly "erased". I display the heart somewhere around the room for the rest of the year as a reminder. The girls seem to remember the lesson.
*We "talk behind each other's backs"...but ONLY in a good way. Each girl gets a chance to sit on my stool at the front of the room, she turns around, and then we all take turns talking about her "behind her back." We say only kind and encouraging things about her, of course. It seems to be a good visual reminder about the kinds of things we should say about one another...even if our friends aren't around!
Hope these things are helpful...best of lucky with your lovely ladies!