DOES preschool work? Although early education has been widely praised as the magic bullet that can transport poor kids into the education mainstream, a major new study raises serious doubts.

Since 2004, Tennessee has offered state-subsidized prekindergarten, enrolling more than 18,000 of the state’s neediest 4-year-olds. An early evaluation showed that, as you’d expect, youngsters who attended pre-K made substantial gains in math, language and reading. But, startlingly, the gains had evaporated by the end of kindergarten.

Those first results were alarming, and worse was yet to come.

A just-released study tracks the same kids to third grade. There’s still no evidence that the children benefited cognitively from preschool. They may be better socialized to school life — a skill, emphasized in preschool, that may well bring long-term benefits — but many of them haven’t mastered the three Rs. That’s terrible news, since being a proficient reader by third grade is widely regarded as the best predictor of high school graduation.

Pre-K critics will again pounce on the results. “Devastating for advocates of the expansion of state pre-K programs,” wrote Russ Whitehurst, a senior fellow at the Brookings Institution, commenting on the first-round evaluation. It’s an “I told you so” moment for Tennessee State Representative Bill Dunn, who slammed his state’s prekindergarten as “like paying $1,000 for a McDonald’s hamburger.”