I don't think our school has ever said anything to DS's classmates, ever. They don't talk about it at all. I *believe* they consider it a HIPPA or FERPA thing, even if they have parent's permission, they don't talk about it to any of the kids. If one of the kids says something, they reply along the lines of "that's just the way he is" and "yes he does move his arms a lot when he's thinking" and then talk about something good that he does too.
I do not know what they do if DS himself tells someone, I should ask.
I never really thought about it. I do remember reading Temple Grandin and how her mom went to the school a day early to talk to the class. And I see both sides of it. At some point the kids notice a difference and when they do, it should be explained properly. But at the same time, giving a label has a downside too, the child being looked at differently.
I did buy a copy of "Everybody Is Different" for the school so that should it come up, the resource is there. It's a pretty good book, written about 3rd grade level.