The Shanker Institute has just released a policy brief titled "The Evidence on Charter Schools and Test Scores", which finds
The available research suggests that charter schools’ effects on test score gains vary by location, school/student characteristics and other factors. When there are differences, they tend to be modest. There is tentative evidence suggesting that high-performing charter schools share certain key features, especially private donations, large expansions of school time, tutoring programs and strong discipline policies. Finally, while there may be a role for state/local policies in ensuring quality as charters proliferate, scaling up proven approaches is constrained by the lack of adequate funding, and the few places where charter sectors as a whole have been shown to get very strong results seem to be those in which their presence is more limited. Overall, after more than 20 years of proliferation, charter schools face the same challenges as regular public schools in boosting student achievement, and future research should continue to focus on identifying the policies, practices and other characteristics that help explain the wide variation in their results.