Teach your daughter some simple relaxation methods. Counting to ten still works well, and taking deep, even breaths can help also. Check in a local bookstore or with your school counselor for books on teaching relaxation techniques to children.
It may help to keep in mind that your daughter must feel comfortable with her family to behave this way at home. She is controlling her anger and her behavior at school and only letting it out at home where she is with people who she knows love her.
If your daughter continues to have problems with this, you may want to talk with the school counselor. He may be able to give your daughter some individual time or include her in a small group on anger management. You may decide at some point that she needs additional help outside the school; if you do, the counselor or your pediatrician can refer you to a therapist in your community.