I was bullied throughout my youth and high school years and I vowed that if I ever had a child (especially a son) that such would never happen to him. My parents often have felt bad because they didn't know how bad it was, because I didn't tell them. I wanted to take care of things myself, but too often my approach of trying to "rise above" and "take the high road" by doing nothing only encouraged them further. Turn the other cheek, yes, but if such only nurtures a passive-aggressive attitude, then we are not actually turning the other cheek but, in fact, cursing our enemies instead of blessing them.
I would teach my child to stand up for himself. However, I would be quick to point out that standing up for oneself can result in being harmed, just as doing nothing can be. But my child, supposing I ever have one, will not be a victim. I won't let him. Hopefully, he will learn to choose his battles wisely.
Telling the teacher or the principal is often counterproductive. As a teacher I can relate why. Teachers identify more with the popular kids and the ones who do the bullying. It's just human nature. The bullies are often the outgoing ones, the ones who stand out (regardless of whether they are intelligent or not) and are seen not as troublemakers but as models. The teachers are more likely to just shrug off incidents and say to "man up" or "toughen up."
I really don't have any solutions, but I know ignoring the problem will not help.