3 things I always told my boys.
1) Don't be an assshole and start a fight or be a bully.
2) You may not win, but make damn sure they would think twice about the next time. It's a fight and the object is to win.
3) Stand up to anyone who is going to bully the little guy.
Said I would back them up 100%. Oldest got into 1 fight. Ended quickly. 1 punch each. Other kid hit first, mine hit last. That was the end of it.
Youngest is too well liked by everyone, including the teachers. I don't think he ever met a stranger. No one has ever messed with him.
Clyde