In my county, it's up to the school. When I talked to the TD department at our School Board they said that technically they could send him back to Kindergarten because he didn't finish (we pulled him in March of last year) and my records of work at home aren't required to count. She said more likely, he would just be put in with his age mates, but it would be completely up to the principal to make that decision.
I would suggest calling the School Board and ask how they handle a homeschool child re-enrolling if the child has been working ahead, since your county might handle things differently.
