I never taught my sons non-violence, and nor did I preach gathering evidence for authorities. I taught them both that the proper response to bullying was to choose the biggest aggressor and then hit him as hard as possible in the nose and balls, to cause as much bleeding and pain as possible. When my oldest son was attacked by a larger boy, and came to me crying, complaining about being hit in the stomach, I told him where to hit the other guy, and after a short hug, made him go and do it. He has been grateful to me for the rest of his life, because that guy was so shocked and scared he never hit my son again. I taught my sons, "If you're hit and fight back fiercely, you will only get in one fight a year--at most. Otherwise, you'll have to take shit all your days at school."
Non-violence is utter bullshit, as a tactic, and the "authorities" are not there to deal with petty issues.