LANSING -- States should limit expansion of online virtual schools until more information is available on why student performance lags behind other schools, according to a new analysis of virtual schools by the National Education Policy Center.

The study examines the changes in virtual school legislation and performance over the last two years in 30 states where the schools operate, and concludes lawmakers in most states have not addressed concerns with how the schools are educating students.

"It [the report] finds that despite considerable enthusiasm for virtual education in some quarters, there is little credible research to support virtual schools’ practices or to justify ongoing calls for ever-greater expansion," wrote the authors.

The report specifically cites Public Act 129 of 2012, which lifted Michigan's cap on virtual charter schools, as being unsupported by evidence showing the schools are effective at educating students.

"This potential massive expansion of full-time K-12 online learning in Michigan was not justified either by the performance of the state’s existing online charter schools or by the existing research into full-time online learning," the report states.

Researchers, including Western Michigan University professors Gary Miron and Brian Horvitz, called on lawmakers to slow the expansion of virtual schools, citing federal education statistics showing poor graduation rates and weak "adequate yearly progress" results.

"On average, virtual schools’ Adequate Yearly Progress (AYP) results were 22 percentage points lower than those of brick-and-mortar schools," the report states. "Policymakers should slow or stop growth in the number of virtual schools and the size of their enrollment until the reasons for their relatively poor performance have been identified and addressed."

Recently released MEAP results for Michigan Great Lakes Virtual Charter Academy and Michigan Virtual Charter Academy, two virtual schools operated by the for-profit company K12, showed mixed results. Great Lakes Virtual had low proficiency rates in most subjects and grade levels, while Michigan Virtual had around half its students proficient in reading at all grade levels, although rates in other subjects were lower.

Brian Smith is the statewide education and courts reporter for MLive. Email him at bsmith11@mlive.com or follow him on Twitter or Facebook.