I struggle with this too. As primary school students a few of us suffered at the hands of a bullying teacher who used to grab me by the back of the hair. I never said anything until one day my mother noticed the redness on my scalp whilst cutting my hair.
The following day, she didn't bother going to the head teacher/principal and all 4'11" of her just marched straight into the classroom, grabbed the near 6' teacher in the exact same fashion and marched him outside in front of all of us, where she proceeded to tell him the way that it would go if he continued.
It was strange to see this as my mother was one of the most gentle, forgiving persons in the world and until then I had never heard her raise her voice, let alone use physical force against anyone. Needless to say the bullying stopped before my father got to hear about it.