I had the same situation with my son. He was academically ready (knew all his letters, most of the letter sounds, basic addition/subtraction and pre-reading), but he couldn't sit still, liked to be the "class clown", etc. We put him in a pre-K program that wasn't super-academic, but did concentrate on "kindergarten" things like circle-time, listening to the teachers and other students, taking turns, following instructions, etc.
When he started kindergarten, I could see kids in his class that were where he was the previous year and it was difficult for them to sit still, follow instructions, etc.
We just had his first grade parent/teacher conference and while he is ahead of many students in his class in math skills and reading, he is socially equivalent to them. He does not get in trouble for impulse control things. Although, he still likes to be "class clown" it only takes one reminder for him to settle down.
And the academic stuff is easy to supplement, he just reads a lot at whatever level he is comfortable with and his teacher gives him extra math homework that he can do to challenge himself. I think it is much easier to make up for the academic difference than it is to make up for the deficiency of classroom social skills.
Just my experience - hope it helps...