Teach a child the to have the confidence to stand up for him/herself to and the knowledge to know how to, and you will find that they seldom will have to hit back because they are seldom ever hit to begin with.
I think children should be taught early that starting fights is unacceptable. But they should also be taught self-defense, because sometimes other kids just don't care. And there isn't always a guarantee that an adult will be available.
I think children should be told it's their call, that there's nothing wrong giving back what you're getting, but there's neither anything wrong in reporting someone attacking you.
Fight is like sex, it's only that if both agree to it, otherwise it's an assault.