Virtual public schools, which allow students to take all their classes online, have exploded in popularity across the United States, offering what supporters view as innovative and affordable alternatives to the conventional classroom.

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Now a backlash is building among public officials and educators who question whether the cyber-schools are truly making the grade.

In Maine, New Jersey and North Carolina, officials have refused to allow new cyber-schools to open this year, citing concerns about poor academic performance, high rates of student turnover and funding models that appear to put private-sector profits ahead of student achievement.

In Pennsylvania, the auditor general has issued a scathing report calling for revamping a funding formula that he said overpays online schools by at least $105 million a year. In Tennessee, the commissioner of education called test scores at the new Tennessee Virtual Academy "unacceptable."

And in Florida, state education officials are investigating a virtual school after it was accused of hiring uncertified teachers; in the past two weeks two local school boards in the state have rejected proposals for virtual schools.

Some states, including Michigan, Indiana and Louisiana, are still moving aggressively to embrace online schools. But the anger and skepticism elsewhere is striking, in part because some of it comes from people who have ardently supported opening the public school system to competition.

"There's a sense that [online education] is a lot more mainstream now and we need to take a closer look at it," said Michael Horn, an advocate of digital learning at Innosight Institute, a think tank focused on education policy. "I don't think we need to put the brakes on completely, but we need tweaks to accountability models, which will slow growth."

Charter school movement Online courses first appeared in public high schools in the early 1990s. They were promoted as a way for students in isolated rural schools to tap into advanced classes not offered in their towns, or for students at risk of dropping out to make up credits.

By the early 2000s entrepreneurs were pitching full-time online schools - perfect, they said, for athletes with heavy travel schedules, children with medical conditions that confined them at home, or almost anyone who found the hustle and bustle of neighborhood schools uncomfortable.

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Online schools face backlash