California's charter schools (search) are 33 percent more likely to meet their academic goals than traditional public schools, a study released Wednesday found.

Classroom-based charter middle schools stood out in particular, with 81 percent meeting the state-set goals for student improvement, compared with 54 percent of traditional schools, according to EdSource (search), a Palo Alto-based nonpartisan organization that studies public education.

Researchers looked at demographically similar students who were at the same academic starting point, using scores from California's high school exit exam and the Academic Performance Index (search), which includes results from several standardized tests.

The study didn't examine what caused the difference.

Caprice Young, CEO of the California Charter Schools Association (search), said the findings illustrated that charter schools -- especially those serving high school students -- "get kids who are far below grade level, and we bring them up faster than noncharter schools."

No differences were seen between new charter schools and those that had been operating for a while, senior policy analyst Brian Edwards said. There also was little difference between schools that were converted from traditional public schools and those that were started from scratch.

California's charter school law, approved in 1992, allows for public schools that are free from many state regulations, in hopes that greater flexibility will bring academic gain.

There are now about 510 charter schools enrolling more than 180,000 students, less than 3 percent of California's 6.3 million public school students in kindergarten through 12th grade.