The first U.S. charter school opened in 1992. Since then the number of charters has grown to more than 4,000 in 40 states, serving 1.2 million students, according to RAND, a nonprofit research organization based in Santa Monica, California.

Students at charter schools graduate and attend college at significantly higher rates than students at traditional public schools, according to a Rand Corp. study led by a Michigan State University scholar.The study, which offers mixed overall results for charter school advocates, comes amid a national debate over President Obama's endorsement of charter schools, which are experimental public schools that operate independently of the local school board.

Obama recently said he would oppose limits on the number of charter schools.Ron Zimmer, MSU associate professor of education, and colleagues are the first to conduct a long-term investigation of graduation and college-attendance rates at charter high schools.

They found that charter students are 7 percent to 15 percent more likely to graduate from high school and attend college than students at traditional public schools.

"These are some of the most positive results so far for charter schools," Zimmer said. "It may suggest that evaluations up to this point have not been broad enough to capture what's going on in charter schools."

Zimmer said further research is needed to explore the factors behind the higher graduation and college-attendance rates.

Other findings from the study, which looked at eight U.S. locations:

• There is little evidence that charter schools are producing, on average, test score gains that differ substantially from those of traditional public schools. Zimmer said much of the previous research has shown similar results.
• Charter schools do not generally draw the top students away from traditional public schools. In fact, Zimmer said students transferring to charter schools generally have below-average test scores.
• Charter schools do not appear to substantially help or harm student achievement in nearby traditional public schools.

"This study provides evidence that charter schools might be moving in the right direction in terms of high school graduation and college attendance, but test scores and other outcomes might not be as promising," Zimmer said. "So policymakers need to think more broadly about outcomes when evaluating how to proceed with charter schools."