Charters remain a popular alternative among parents to traditional public and private schools, and like private schools, many charter schools have waiting lists for student admissions, she adds.

However, charter schools that rely only on public funding are struggling to the point where some are being shut down, she says. Publicly funded charter schools must submit to state standardized testing and improve testing scores every year; if not, their funding gets cut.

Kendra Bostick, schools program leader for ThomCo, bought this year by Markel, notes that 500 new charter schools were added during the 2011-2012 school year, the largest growth for the sector within a single year.

According to the National Center for Education Statistics (NCES), student counts in charter schools more than quintupled—from 0.3 million to 1.6 million—from 1999-2000 to 2009-2010 (the years used for the NCES study). The percentage of public schools that are charter schools also increased, from 2 percent to 5 percent, or 5,000 schools, the study says.

But some insurance companies may not consider charter schools an attractive risk because they require a five-year start before they can be termed successful, says Shelley Levine, executive vice president for the School Insurance Group at Bollinger Insurance.

“Generally, you're working with a startup from an underwriting point of view: It has no experience, no history of controls behind it, and it's not a favorable risk,” Levine says. “They're also often working on a shoestring budget.”