You are right. The first boy had no business teasing. He will most likely think twice before doing it again. The second boy should try to have dealt with the boy in a different manner first. These days kids can be so cruel. They will pick on who they can pick on. I understand why kids have to stand up for themselves. If they don't, they will most likely hear the same thing tomorrow.

That being said, punching is not always the answer. I taught my kids to try to work things out without ever having to touch someone.

Some parents see no wrong in their child. If you have talked with her already, you may as well stop trying. I am sure she thought more about the situation after you left. If she is set in her ways and believes her child did no wrong, perhaps you could contact the school or the bus driver. A close monitoring over the boys is probably your best bet. The bus driver is the one who was there. He may suggest the boys sit at different ends of the bus so he can drive safe.
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I agree with all of the above. Maybe first it's teasing, later it's bullying. Draw the line quickly and let the kids know that this is not the behavior expected. Not teasing, not bullying, not scratching, not hitting, nothing.

Having said that, I still think that if a child misbehaves, it's the entire fault of the parents. If kids are allowed to watch cruel films and are allowed to play cruel games, what do parents expect? Our brains cannot distinguish between reality and "fake". Subconsciously it's reality for the kids even if they are taught to behave "nicely". Give a bad situation and they pull what they see on tv and internet as a resource and do what they think their hero would have done. Or what they have seen other kids doing in similar situations.

And if the mother sees no wrong in her child ever, then maybe she doesn't know better herself. I am so amazed how many mothers are out there these days who have no clue about how to educate their kids, so they let them do as they please.

Remembering my childhood, we really were quite nice kids in our neighborhood except a few boys that were older than us. I never told my mom, but those boys simply bullied us day after day. One day I was ready for them and defended myself and my friends. The bullies left us alone from this day onward. So maybe one should let kids go to a self-defense club these days where they can learn how to defend themselves when bullies and teasers won't listen to words.

I do think that the kids should be talked to and encouraged to apologize and work this out. I assume by your original question that one of the mom's is dismissing this as harmless and wants to believe her child is blameless. That sounds like a pretty natural reaction, but she's not doing her kid any favors by ignoring the chance for him to learn how to be a better citizen. Harassing others is not acceptable and violence is not an acceptable response. All of the kids would benefit from learning the lesson that you treat others with respect and dignity and that when you feel wronged, you take the high road anyway.

I think a good way to talk to a parent like that is to recognize her point of view, demonstrate empathy, and then make a calm statement about your perspective. Such as "I understand that Johnny felt threatened, and that must have been really frustrating for him. I respect that you want to protect him. At the same time, his choice to get violent is concerning to me and I think as adults, we owe it to all of the kids to intervene and help them to make different choices next time." If one of your kids was involved, recognizing his responsibility as well will go a long way.