The charter school movement has expanded over the last 20 years largely on this promise: If exempted from some state regulations, charters could outperform traditional public schools because they have flexibility and can be more readily tailored to the needs of students. Another selling point is that these schools are supposed to be periodically reviewed when they renew their operating permits — and easily shut down if they fail.

It has not worked out that way. Despite a growing number of studies showing that charter schools, financed with public money and operating in 40 states, are often worse than traditional schools, the state and local organizations that issue charters and oversee the schools are too hesitant to shut them down. That has to change if the movement is to maintain its credibility.

A new study from the National Association of Charter School Authorizers, a nonprofit, pro-charter school organization, found that a smaller and smaller percentage of schools are being denied charter renewals.

According to the study, charter authorizers who oversee many of the nation’s approximately 5,600 charters have, in recent years, shut down fewer schools. Only 6.2 percent of those that came up for renewal in 2010-11 were shuttered, down from 8.8 percent in 2009-10 and 12.6 percent in 2008-9.