I'd say that either personality has its pluses and minuses in terms of a gradeskip.
More reserved kids can be "blenders" who don't make waves with peers, and within very short order, regardless of their obvious physical young-ness, go nearly unnoticed as "different" by peers.
More socially gregarious/assertive kids tend to read as less mature by peers... BUT... those are also the kids (IMO) that seem more likely to get their needs met. But that is true in almost any setting.
I imagine (having only the former type) that one probably winds up spending just as much time on the phone and at the principal's office either way, but in the latter case it's for different reasons than the former. With the former, you'll largely find that administrators (and even many teachers) "don't see a problem" when your child is doodling and reading silently all day, day after day, and still pulling in 100% on everything. The children themselves don't make waves, and they aren't a problem for the teacher* either, so the thinking is often that you must be a TigerParent from Hades.
* the exception, of course, when you have a stealth disruptor for a child in this situation-- that's the kind that encourages classmates to do things that s/he wouldn't dare... and those classmates DO those things. There are many leadership styles that work, let me just add here, and introverts have their own ways about it.
(Don't ask how I know that one, either.)
