Debi, I know the point of the story here is about how your son stood up for himself and I'm loathe to take away from that. However, I'm struck more by the idea that the principal was fully aware of this bullying issue and apparently had been for some time and yet it took your child's own actions to put a stop to it.
In a somewhat related story of my own. We had a similar situation many years ago when our son was in preschool. My son had been quietly playing near another young boy was also playing by himself. Two older kids snatched the toy away from the boy who is not my son and started to play "keep away" throwing it back and forth between them. A teacher across the play room saw this and started walking over to deal with the situation. She said that she only got about 2 steps before seeing my son stand up, grab the toy away from one of the older kids, shove him to the ground, give the toy back to the original boy and then sat back down to keep playing quietly by himself. The teacher told us that she disciplined the two older boys and informed their parents, however she had not said anything to our son and let us know she was secretly proud of him for standing up on someone else's behalf.
More recently, just a few months ago, my son's teacher talked to my wife who is also a teacher at the same school. My son is in 8th grade and often complains about being in class with a bunch of trouble makers who don't want to learn, worse is their antics keep him from learning. On this particular day, the teacher had to take a student out into the hall for a talking to. Apparently, seeing a classmate getting in that kind of trouble had no effect on the rest of the trouble makers as they simply saw it as a opportunity to try things without a teacher in the classroom. One kid got up and started messing with stuff my son was working on, grabbing things off his desk and making lewd comments. My son first said "Please leave me alone" and when that didn't work he tried "I said Stop!" which also didn't work. The teacher said she walked back in the room just as my son yelled out "Get the F--- away from me!! Right Now!!" The teacher has told us that our son is one of the most well behaved, mild mannered students she has ever taught. Seeing our son's outburst, knowing his personality and the personality of the kid getting yelled at, the teacher immediately took my son's side and asked the other student why he was up out of his seat and causing so much trouble that another student has to yell at him for it. This teacher told my wife that she was leaving out my son's uncharacteristic F-Bomb in her report to the principal and only including that other students are having to yell at this trouble maker for his behavior. Due to past behavior and this incident with my son, that trouble maker was no longer a student three weeks later.