Good kids don't "fight". It's so ingrained into the head of the good kids that they are terrified of fighting even to defend themselves.
Thus, bullies get to bully with no fear. There's no consequence. Whereas when a weaker kid fights back, even if the bully is strong enough to "win" the battle, he still gets hit in the mouth and does not like it. Bullies much prefer low-hanging fruit that doesn't fight back.
My children, when school age (maybe not 1st grade, but certainly by junior high), will be sat down and have the following explained to them:
"You are allowed to defend yourself. You will avoid physical conflict whenever possible, but should you ever be physically threatened or subject to ongoing torment, you have the GREEN LIGHT to use physical force to protect yourself, OR to assist a friend who is unable to protect themselves."
"You may get in trouble with your school. THIS IS OK. Your well-being is more important than their rules. If you get suspended for three days, then I'll take three days off work and we'll keep up with your studies. I will be on your side. Do not let concern over the school rules stop you at all from defending yourself."
"However, you will never use force to do anything but protect yourself or your friend. If I find that YOU have been the aggressor, I will smite you."
I'll probably need to work on that speech a bit between now and then, but you get the idea. :)
I was lucky. I was never the small or weak one. Even still, I can think back to a couple of situations where fear of parental response prevented me from protecting myself as well as I should have. My children will have it made known to them that they will not be "assumed guilty" when a situation comes up where they physically defend themselves. No aggression towards others will be tolerated, but they will be trusted to properly use their discretion, and until they prove themselves guilty, they will be assumed innocent.