A study out this week shows that charter school enrollment continues to increase both nationally and here in the Twin Cities.

For the first time, more than 20 percent of all public school students in Minneapolis and St. Paul attend charter schools, according to a study by the National Alliance for Public Charter Schools.

That's certainly not the highest percentage in the country - that title goes to New Orleans where about 70 percent of all public school students attend charter schools. In Detroit, that number is 51 percent and in Washington D.C. it's 43 percent.

The study shows that - over the past year - charter school enrollment has grown in Minneapolis by 7 percent and by 10 percent in St. Paul. One in five students in the Twin Cities attend a charter school, according to the report.

Joe Nathan, executive director of the St. Paul-based Center for School Change, said the growth in the Twin Cities charter school enrollment can be credited to the expansion of existing, successful schools like Harvest Prep and Hiawatha Academy and the addition of new schools.

"Part of what we see in more successful schools - in terms of test scores - is them expanding in their own building or adding a new building," he said.

Other key findings in the report include:

• In Los Angeles more than 120,000 students attend public charter schools, which is more students than are served by 99.9 percent of American school districts.• In 135 American school districts, at least 10 percent of public school students attend charter schools.

• In the five school districts with the largest growth in charter school enrollment, charter enrollment grew by an average of 35 percent in the 2012-13 school year, adding nearly 14,000 new charter students in those communities. Those school districts were: Hall County, Ga.; San Diego; Duvall County, Fla.; Newark, N.J.; and Hillsborough County, Fla.

• Four school districts have been in the top 10 for growth two years in a row: Hillsborough County, Fla.; Los Angeles, Boston; and New York City.

Joe Nathan