I agree with what you are saying Jane but I think you are only looking at it from a social perspective. Sure many 9 and 6 year olds can play together. They may follow complex rules, play a game of tag, understand when to stop etc- far better than a 3 year old would be able to follow the game. But it's a rare 9 year old that has the patience to work on an academic project with a 6 year old.
Now add in that we're talking about a theoretical 9 year old, who is actually 6 and doesn't like slowing down for his friends, helping others always or spending 80% of his time rehashing information he already knows. This child either gets frustrated by the constant drag or begins to pretend that he doesn't know the information just so it's not his responsibility so often.
To take the analogy one step further. If you are the teacher and you have the same 9, 6 and 3 year old in class- you will likely group the 9 and 3 year old together because the 9 year old can help the 3 year old learn a LOT! Oh and it'll be good for the 9 year old too to learn some patience.
Dottie's right, a 3 deviation below child wouldn't likely be in a typical classroom full-time and if one were, s/he would have a full-time aid.
Maybe that's what I am missing, my kid needs a full-time aid! :-)
