Montgomery and Fairfax county schools continue to lead the nation in high school graduation rates, according to a ranking of the largest school systems made public Tuesday.

About 86 percent of students in Montgomery and 85 percent in Fairfax graduated on time from high school in 2008, the most recent data available, up from about 83 percent in both counties the year before. The four-year graduation rate in Anne Arundel County was 76 percent, putting it in sixth place. Prince William County came in 17th with 68 percent, and Prince George’s County 20th with 67 percent.

The average rate nationally was 72 percent, up from 66 percent a decade before. The lowest graduation rate in a large school system was in Detroit, with 33 percent.

States use varying methods to report graduation rates. The list made public Tuesday was part of an ongoing analysis of graduation trends by Bethesda-based Editorial Projects in Education Research Center and published in Education Week.