I think there is a great deal of leeway in kindergarten with the expectation that the child is learning to do school.
By mid-first grade I think the expectation is a more in terms of getting along with others, following direction, not interrupting, etc.
I think it does make it difficult for a summer kid who intellectually needs the next grade but is not developmentally ready for the expectations.
I also don't think that's the only thing to take into consideration for skipping...
I'm sure behavior won't be an issue for K., but I can't imaging a HG/PG having a really good experience in K.
Last year was a disaster for us.
And our oldest daughter didn't have the same terrible experience, but it just wasn't a good fit.