Hmm... perhaps perfectionism also rears its subtle head... when you place a high value on obeying social mores and combine that with some perfectionism... the more complex the social environment the longer before you are ready to enter. Even more complicated for a kid when other kids are following mutable social rules.
A not quite introverted, not ASD situation that may be quite common amongst gifted? Even worse is if you have a high degree of empathy and read frequent disconnects between what is said and done and what is felt.
This is EXACTLY my son... plus he is just intorverted by nature as well. He's so much more complicated than your average kid. And, yeah there is truth to what Polarbear said - of course. BUT A LOT of teachers just aren't very good and don't understand (or take the time to understand) a more intelligent/complicated kiddo.
