There's a new study titled "Charter School Achievement: What We Know" by Bryan C. Hassel and Michelle Godard Terrell for the National Alliance for Public Charter Schools. The study summarizes and evaluates 58 charter school studies.
Recommendations from the study include:
1. We need better research about how well students in charter schools are performing.
2. We need more and better research about why some charter schools perform so much better than other charter and non-charter schools.
3. We need much more attention focused on evaluating chartering as a policy. Knowing how well charter school students on average are performing does not answer the most important questions policymakers have about where to go with their charter policies.
4. Charter schooling represents an experiment worth continuing -- and refining to improve quality further over time.
In general, the study determined that studies really needed to compare how students did over time rather than how students in charter schools are doing to students in a comparable non-charter school.
The charter is but a shell, into which the operators place an instructional and management program. Asking about the quality of "charter schools" as a group is a bit like asking about the quality of "new restaurants" or "American cars" -- any overall generalization will mask the great diversity within.