My two cents: You may choose to contact the teacher and ask her to monitor the situation and contact you with more information about the schools point of view. Sometimes they can give you information about the dynamic that is more objective. However, I would leave your questions about the other boy’s giftedness out of the conversation completely.
From what you have written, it looks like the other boy has poor social skills (which is very common with boys!) and was picked on himself. The most I would say about that to a teacher is that research shows that kids who are bullied often do respond by bullying others.
Does your son have a friend or friends on his side? Is he depressed or refusing to go to school? If he has no friends at all and is starting to refuse school, then this is pretty serious and I would definitely step in. If he has friends and is okay with school, maybe it is something he can be coached to respond to himself.
