When I was being bullied at school, over 20 years ago now, and eventually told my mother, she took me to see the principle, showed him the bruises over me, and then the matter got sorted.
Boy was talked to, as were his parents, and it stopped.
Several others in my class thanked me afterwards as he stopped beating the ****e of them at the same time.
Best decision I made was to tell someone.
It was a different situation to boys mucking around and beating each other a bit as you tend to do, in that situation best thing to do is thump back, but aim to land one on a hard spot, or throat. Generally that way they'll swing for someone else first the next time a beating session starts.
Only other occasion I properly recall was when the biggest bully in the year thumped me on the back of the head as we were walking into the locker room one day, he was in a different class, I actually thought it was a m8 arsing about, spun round and grabbed him hard by the throat (the only ninja-esque move i've made in my life, I truely think I was a blur in that moment) then my brain caught up and I realsie I had the hardest most evil ******* in the year by the throat.
Several seconds seem to pass in slow motion as I pondered just what way I was going to die... but no, he smiled, a big broad smile, I smiled and let go of his throat, we laughed and walked off to class. At that time I really didn't understand why I was in one piece.