We didn't have kids so near in the neighborhood, so we didn't tell many people except the friends who know him best and were not surprised. we just waited for questions after school started and said that the principal and the teachers thought it was more appropriate. But our DS was younger--skipping 1st. I would say in your case keeping it simple would be good. E.g., "just wanted to let you know that the school thought that 7th would be a better fit for DD. I'm glad that she has such good friends nearby in the neigborhood so it will be easy to stay friends after school."
I think this is the right approach. We did just move here in March, so she is not close with the other 6th grade girls in the neighborhood. (the girls at school she hit it off with are going to the other middle school) They don't seem to have much in common and I have seen them walk right by my daughter and not say hello. (I don't think they are deliberately being mean--just kind of happy with their own group and have not learned or been taught how to include someone new) This is just another reason I am happy for the skip! It makes it less wierd that she isn't really friends with them if she is in 7th.
