According to the Global Report Card, more than a third of the 30 school districts with the highest math achievement in the United States are actually charter schools. This is particularly impressive considering that charters constitute about 5 percent of all schools and about 3 percent of all public school students. And it is even more amazing considering that some of the highest performing charter schools, like Roxbury Prep in Boston or KIPP Infinity in New York City, serve very disadvantaged students.As impressive and amazing as these results by charter schools may be, it would be wrong to conclude from this that charter schools improve student achievement.