Seeing all these different opinions is very interesting to say the least. As a 15 year old, I've had plenty of bullying in the past. IMO, if it turns violent, and you are stronger than the bully, then beat him up, or at least scare him.
I once had a bully who kept on harassing me and pretty much everyone in the class. When I told him to stop it didn't work, when the teacher told him to stop it didn't work, so I had no choice but to stand up to him. Heres how it went, he started harassing me by hitting me with this tiny stick thing (it was during the environmental education class), and I snatched it out of his hands, so he got another one of those sticks and started hitting me again, the cycle repeated until he was eventually corner in the class shed. Then what I honestly didn't expected happened. He grabbed a pair of those giant gardening scissors and pointed it towards me, saying :" Whatcha gonna do now ? " He could've easily caused some serious damage if it was somebody else but thankfully the bully was slow and inexperienced with fighting. It took me less than 1 minute to take the gardening scissors from him. When I got the scissors off of him, I chose to throw the scissors away and stared into his eyes. That immediately crushed his confidence and he ran away saying :" Miss, he is trying to hurt me. "
Later on, he was shameless enough to challenge me to a fight, being the teenager I am, I obviously accepted. Instead he never showed up.
This just goes to show bullies are actually a lot weaker than what they want you to think they are. If someone think that I'm just acting the way I am because I have a physically advantage, heres another story. I grew up with a violent dad and what he does to me is nothing short of bullying, I was not physically stronger than him. We still shared a father and son bond and that wouldn't change, but physically discipline (or so he says) me for not getting good grades is just ridiculous, especially considering I skipped a grade due to moving to a different city, but eventually that stopped too, after I told my dad the next time he hits me, I will protect myself by hitting back, even though I knew he will beat me up.
TL;DR version, if you don't stand up to the bullies, they would only stop when they feel like it, they need to be taught that they can't have their way with you.