No personal parenting experience (yet!), but happened to have a conversation this weekend with my stepmother about HG stepbrother in the context of teaching social skills and she mentioned it was hard to get him to see the possibility that he might not be the expert on something, or that he could even be (gasp!) wrong sometimes and she really had to work with him on that.
So, although it could be a condescending attitude or the way that another mannerism is perceived as rude or arrogant, it might be things you haven't considered like over-sharing of his expert knowledge on the topic or a habit of "helpfully" correcting people that makes him present this way. I can see this eventually being a problem for my DS5, because in our house we tend to have lively discussions on all sorts of topics and he is allowed to ask challenging questions and point out inaccuracies to the adults (for the record, I'm usually wrong) which may not be generally acceptable behavior. He recently accosted a museum docent with intense questions and his father and I had to move him along in order to rescue the poor overwhelmed lady.
As a child I was perceived as "stuck-up" when the reality was that I was extremely shy and I really had to learn how to affect a "friendly" or more outgoing demeanor as an adult. Oh, and then there was the older boyfriend in HS who broke up with me because I used words he didn't know and that made him look dumb in front of his friends. That was me "thinking I'm smarter" than other people too.
I guess what I'm saying is there are lots of gifted behaviors that can be taken as offensive to others but aren't necessarily seen that way in a group of other gifted people. I'd ask the teacher for specific examples of things he does or that other children say which made her feel it is worth mentioning. Maybe she won't be able to pinpoint a specific example. In that case, ask her to mention it to you next time something happens so you can address it. You may get a better idea of the "problem" and how to deal with it.
