When compared to the School Performance Profile, the average score for “traditional public schools was 77.1, but for charter schools, it was 66.4, and cyber-charter schools came in at a low 46.8” (Are Charter Schools the Answer?). The purpose of these schools is to simply be the top academic schools. From this point of view, charter schools are not only lower than the standard score of 70, but they are doing worse than public schools.
Found some ugly numbers out there, and a well written article about this here.
https://medium.com/fhsaplang/charter-schools-the-demise-of-public-education-2f3ce3a5725a
There's some citations of course...