One day my kid was in third grade (about a month into school year). That night we got the call that they were offering a whole grade skip and if we accepted it they would move him the next day. We were outside the front office waiting to talk to the guidance counsellor and my son told one of his third grade classmates (they had known each other two years) he was moving to 4th. The classmate said very genuinely and supportively "that will be so GOOD for you!" That set the tone for the whole skip and was the best thing my son could have heard.
I say that the kids who know your child and genuinely care for your child will understand intuitively just like this little friend did. It wasn't a shock (even though skipping rarely happens here) because kids know what the deal is. I don't know about how the classmates of the class he moved into felt but it worked out okay.
