Last June Stanford U's CREDO published the first nationwide assessment of charter school test performance. Here is the Press Release from the researchers of that study:
Stanford, CA – A new report issued today by the Center for Research on Education Outcomes (CREDO) at Stanford University found that there is a wide variance in the quality of the nation’s several thousand charter schools with, in the aggregate, students in charter schools not faring as well as students in traditional public schools.
While the report recognized a robust national demand for more charter schools from parents and local communities, it found that 17 percent of charter schools reported academic gains that were significantly better than traditional public schools, while 37 percent of charter schools showed gains that were worse than their traditional public school counterparts, with 46 percent of charter schools