Mosle, who once riled teachers unions herself, points out that, well, things are complex. She ponts out that while teacher quality may be the most important variable within schools, there's plenty of research showing that that "most of the variation in student performance" is a result of factors outside the school: "not just poverty, but also parental literacy (and whether parents read to their children), student health, frequent relocations, crime-related stress and the like."
As a teacher herself, Mosle points to one well-known study from 2009, which surveyed approximately half of all charters nationwide and was backed by the pro-charter Walton Family and Michael and Susan Dell Foundations. The study found that more than 80 percent of students either do no better, or actually perform substantially worse, than traditional public schools—not a great record and perhaps a reminder that charter schools were not started to outperform traditional schools necessarily. The study concluded that “tremendous variation in academic quality among charters is the norm, not the exception.”