Random thoughts triggered by Homer's puberty tale.
My son is now 6'3 and appropriately broad, but he was a skinny introverted kid that attracted the attention of bullies. We put him in aikido for a year where he was the only non-adult at 13. Gave him confidence and poise, which was what we intended. The main middle school bully was one of those big kids with knuckles that dragged the ground and a single eye in the center of his forehead. One day Son and two of his skinny introverted friends were waiting for the bus when Bully and two of his henchmen approached. Bully grabbed Son's arm. It wasn't a friendly grasp: his fingers left marks that stayed for several days. Son said, I don't want to fight; let me go. Bully replied, who's gonna make me? Son's friends told me later that they knew Bully as going to hurt Son, so they were going to have to leap into save him, and then they'd all die. Son made one more polite request, Bully got more aggressive. Son said, "Mom, I just wanted to break his hold on my wrist, but when I made my move, he (Bully) went UP in the air and came DOWN on his back." Bully lay still a moment, turned over, crawled away, got to his feet and left with the henchmen, never to be seen again.
We told both our kids, that if they ever started a fight, they would be grounded for life, but if they ever fought to defend themselves or someone else, we'd back them all the way to court, if necessary.