I wouldn't say anything, myself, as they will certainly notice it for themselves sooner or later. But DS's neighborhood friends are all up and down the grades, so it didn't really make any difference when he skipped K, and he hasn't skipped anything since. We may have a skip after next year, or the year after, but everybody knows him and wouldn't be surprised. He has three neighbor friends a year younger than he is, who are one and two years behind him in school due to cutoff dates, and one a year older who is in his grade.

When they notice the skip, and ask questions, that's when you can do the "oh, yes, we and the school felt it was best, please pass the mustard" thing.