South Carolina public schools had the worst high school graduation rate in the nation in 2002, according to a new report from The Manhattan Institute, a conservative think-tank that supports school voucher programs.

The study says 47 percent of South Carolina students dropped out before graduating in 2002. In the same report, authors Jay P. Greene and Marcus A. Winters conclude that just 29 percent of South Carolina students who enter public high school end up qualified for admission to a four-year college.

The state Department of Education agrees that high school graduation rates are too low but argues that it's not fair to rank states by graduation rate without considering graduation requirements.

"No state has set the bar higher than South Carolina," said departmental spokesman Jim Foster. "If we had North Dakota's standards, or Wyoming's or California's, a lot more of our kids would be graduating in four years."

He said South Carolina is one of only six states that require 24 credits for high school graduation, and is among the roughly half of the states that require high school exit examinations.

Foster said that, of students who graduated South Carolina public high schools in 2002, more than 42 percent were enrolled in four-year colleges the following year.

The Manhattan Institute's authors used their own formula for calculating graduation rates, rather than relying on government statistics they believe are inflated. The study put South Carolina's 2002 graduation rate at 53 percent, the lowest in the nation and the state's worst showing in the 12-year period studied.

Foster said the state's calculations put the 2002 graduation rate at 67 percent, but he's quick to say that's an unacceptable rate as well.

The Manhattan Institute study, expected to be released today, could provide fresh ammunition for Gov. Mark Sanford and supporters of his Put Parents in Charge school choice initiative. A rally in support of the initiative is scheduled on the steps of the state Capitol this morning.

Winters, one of the study's authors, said he was unaware the rally was scheduled the same day as the distribution of the report.

The report looked at schools across the country and concluded that the national high school graduation rate for all public school students declined slightly from 1991 to 2002, from 72 percent to 71. At the same time, "the percentage of all students who left high school with the skills and qualifications necessary to attend college increased from 25 percent in 1991 to 34 percent in 2002," the report said.

In South Carolina, however, the graduation rate fell from 65 percent to 53 percent during the same period, while college readiness increased from 24 percent to 29 percent.

A different study by The Manhattan Institute's Greene last year said South Carolina's public schools perform better than expected, considering the disadvantages students bring to the classroom. At the time Greene said that South Carolina was among "places getting a bad rap undeservedly."

Foster said the state has been focusing its efforts on early grades, because research has shown that's where schools can make the greatest difference. He said that, in contrast to the study's suggestion that South Carolina has the highest percentage of dropouts, federal figures rank the state seventh best.

Winters said The Manhattan Institute study doesn't count people who earned GEDs as graduates, or people who completed their high school diplomas in adult education classes.

"In our method, you're either a dropout or a graduate," he said. "We think that our rate (calculation) is the best."

Winters said differences in state graduation standards aren't factored into the study but probably wouldn't change the rankings very much.

"The standards do differ between the states, but they don't differ that much," he said.

The report doesn't point to a solution for improving graduation rates and school readiness, but earlier reports by Greene have concluded that state support for private schools, charter schools, home schooling, and choice among public schools improves performance.

"We do believe our own research, and our research supports school choice programs," Winter said.