Well, when I was a child, I regularly stood up for people being bullied, and the bullying stopped. Now, I was a child at the time, and of course you aren't. That certainly changes the dynamic. But in response to your question, yes, I can say for sure that "This happened, I said something, and it stopped." *Many* times!
By the time I was 7 or 8, I'd say, pretty much everyone in the school knew that I would put a stop to any bullying that I saw, and I didn't care what the bullies thought of me or said about me. I was never in a fight of any sort either. People just stopped because I told them that what they were doing was mean and I wouldn't allow it to continue. I used the threat of telling the teacher, but I very rarely had to actually do it.
Was I the most popular kid on the playground? Not with bullies. But since I wasn't too keen on them either, that was okay by me. Before very long, I had a substantial circle of friends who stood by me when bullying happened because they knew I was there for them, too. It's easy to bully one person alone, but when people refuse to be bullied and stick together, it becomes much harder to bully them.
However, since you are an adult, I'm not sure any of this is relevant to the specific case at hand.
Instead I think you are in a position of authority, which makes everything different. I wouldn't approach it as defending your child. I'd approach it as teaching the kids a lesson in manners that they sorely need, regardless of whose child they're talking about. I definitely wouldn't get upset. I'd just very calmly tell them that they need a lesson in manners and that their parents must be very disappointed in their behavior. I wouldn't be angry; I'd be sternly parental or teacherly!
But again, that's my personality. Heck, if I were on that bus and saw those boys being mean to *your* child, I'd speak up and tell them to cut it out. I'm not sure I'd be able to help myself, frankly!
I realize that's not going to work for everyone...
