I agree that her choice of words was very poor. Teachers are human, and they have bad days, and hey, some of them just aren't as good at their jobs as others.
If it were me (my DS8 is in 3rd grade also), I'd talk to him about how even adults sometimes choose words poorly, and explain (like MK13 said) that she should have talked about the behaviour and not him. Also remind him that when a teacher is talking, the students should stop talking so that everyone can listen, etc etc.
To me this is a perfect opportunity to teach him about effective communication, ie "what could the teacher have said that would have been more accurate? You know you're not a bad boy, but you should have been quiet while she was talking. Her words weren't very clear, were they? What are some other words she could have chosen?" etc.
Another advantage to this is that next time it happens (because it will, because the vast majority of people out there are imprecise with language) is that your son my reflect back and assess the situation objectively, rather than thinking he's a bad kid (and as a result be more likely to remember how to behave next time).
Turn it into a life lesson
