I should mention (duh) my DS5, who probably would do poorly in a traditional K class, has trouble with sound-letter association (speech delay doesn't help), and fine motor problems, is a visual-spatial learner with auditory-sequential weaknesses, in addition to being very introverted (has basically no buddies except for his twin brother. His twin brother has several buddies). Also, he's quite small for his age. On the surface he'd seem like a classic case for redshirting, if it weren't for the fact that he's a grade level and a half, or more, ahead in math.
We have, temporarily at least, solved the problem that a traditional school structure wouldn't work for him - he's in a montessori school with a young upstart teacher who gets him and who is very flexible and open to new ideas, and he's doing great. (He has the same teacher/classroom for K as he had for preschool - 3 grade levels in each class. But the K is full day, whereas the preschool was half-day) He starts K in 11 days...