Going into middle school (or any grade level.) I would add that words can hurt even more when they are spread via text or computer.
I'm not a Christian but the best thing ever said was "Do unto others as you would have them do unto you." Tell your child, "If you wouldn't want this said about you, don't say it about anyone else."
I used to tell my sons, "You do not have to like everyone but you do have to be civil. Don't like someone? Ignore them." I know, not always easy to do.
Last thing, remembering the words of Michelle Obama, "When they go low, we go high."