Ms. MillCreek, the elementary school teacher, has had a couple of unschooled kids over the years. The parents either got tired of unschooling or the kids themselves asked to start going to school.
In her experience, both kids had some significant academic and social skills delays compared to their age cohort. Although by the end of the year, they had pretty much caught up. Both of these kids came back into the school system at third grade, she thinks that there are more academic delays the longer you wait.
She is not a fan of unschooling, for pretty much the same reasons articulated elsewhere in this thread. She is a fan of homeschooling for the right students, the right parents and the right curriculum.