Secretary of Health and Human Services Kathleen Sebelius recently remarked,

We’ve got to make sure that kids by the time they hit kindergarten aren’t so far behind that they don’t ever catch up, and by the third grade they may as well drop out because they’re never going to catch up.
Of course, Secretary Sebelius is trying to make an argument for expanding government-funded preschool, even though some studies suggests that preschool benefits disappear by the third grade (for most students).

That issue aside, one would hope that children who enter kindergarten with educational and social deficiencies would receive intensive instruction in grades k-2 to address those concerns.  If after three years of schooling the child has not caught up to his or her peers, who should get the blame – those who failed to provide one year of preschool or those who provided subsequent years of instruction?