You've received great advice. I agree with polarbear about talking with your daughter, and with ConnectingDots that books can be a great way to open those conversations.
The American Girl "Smart Girl Guides" include titles such as
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Drama, Rumors, & Secrets: Staying True to Yourself In Changing Times,
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The Feelings Book: The Care & Keeping of Your Emotions
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Friendship Troubles: dealing with fights, being left out & the whole popularity thing
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Know What to Say: finding the words to fit any situation
Rosalind Wiseman's
Queen Bees and Wannabes is an in-depth look at relation aggression.
