BOISE — A new report takes aim at the nation's largest for-profit online education provider and finds students taking K12 Inc. classes in Idaho and four other states are falling more behind in math and reading than their traditional school counterparts.

The study was released Wednesday by the National Education Policy Center at the University of Colorado Boulder. Along with lagging test scores, the report says the rate at which K12 students graduate on time is far lower than in regular schools.

The study "into K12 Inc. raises enormous red flags," said center director Kevin Welner.

The group has previously issued reports critical of online learning. A study released by the center in October said school-choice advocates are pushing states to rush headlong into virtual education despite limited data on these programs.

The latest report looked at schools managed by Virginia-based K12 in Idaho, Arizona, Arkansas, Ohio and Pennsylvania and found that on average, they had a consistently lower proportion of their students meeting or exceeding state standards in reading, according to 2010-2011 test scores.

The report also said math scores were lower compared to the state average. The on-time graduation rate for K12 students was about 49 percent for that year, compared to about 79 percent for the states, according to the study. In Idaho, where K12 provides curriculum for the state's largest online charter school, state officials say students beat slightly the statewide average for graduation rates in the most recent year.

K12 has managed online schools in 29 states with mixed academic success.

The company contends that the report is flawed and fails to show the academic progress of students over time. The report finds K12 students are falling further behind in math and reading than students in regular schools, but doesn't provide evidence to back up that claim, the company said.

"To make such conclusions, one would need to know the academic starting point of the students, in this case, test scores from a prior school year when they were enrolled in a brick-and-mortar school," the company said. "That test data is largely unavailable and is certainly not included in the NEPC report."

The report relies on "static, end-of-the-year test data," said the company, which also noted that K12-managed schools tend to enroll students who are behind academically.

More schoolchildren than ever are taking classes online.

The debate over virtual learning has become heated in states like Idaho, where students have to take at least two credits online to graduate high school, under education reforms that were approved in 2011 and will go before voters in November.