This is a topic of major importance to me that caused quite a bit of ranker in our family and extended family. My kids were raised in an environment where they were a minority and subject to racial, cultural, a economic prejudice. Kids of our background were routinely bullied, intimidated, and physically harassed. As early as 5 years old... that's right... kindergardeners.
In preschool, the children were not yet able to differentiate between races or socio economic, or cultural differences so the battles on the playground had more to do with possession of toys or accidents.
As early as kindergarden, though, my kids came home and confided with us that had been attacked by other kids because they were a different race, or because they had different clothes or spoke differently. I went to the school and told the teachers that I gave permission to my kids to defend themselves. That they should never start a fight but that if they were physically attacked by anyone, they had my permission to do what ever was necessary to stop the attack. That I had taught my kids how to punch, kick, bite, and scratch and where to apply those techniques. I told my kids they had nothing to fear from me, that I would defend them no matter what the school said or did. The teacher said that was not acceptable. I said that was the way it was going to be as far as I am concerned and walked out.
Funny thing, after that, my kids had very few problems with the other kids at school... sure there were the occasional incidents... but for the most part, parents of the other kid with a bloody nose or a bruised lip met with us in the principals office and it became clear where the prejudice had originated from.
I am not saying this is the solution for all circumstances and that it could have turned out differently... but for me and our situation, it worked out ok. My kids were pretty easy going... I couldn't get them interested in martial arts... one is now an actor the other an artist... But I did not want them to be intimidated or broken down by ignorance or prejudice... that chain must be broken... and I think they did a great job of it.