Fairfax County has knocked Montgomery County from the top spot on a list analyzing graduation rates for the country’s 50 largest school districts.

Graduation rates in Montgomery slipped more than 3 percentage points, to 84 percent from the previous year for the Class of 2010, according to a report from Education Week released Thursday.

Montgomery had been in the No. 1 spot of the publication’s “Diploma’s Count” report for the past four years. It is now tied for second with Baltimore County.

Montgomery officials say they don’t like the drop but are still happy to have some of the highest graduation rates in the country.

“It is disappointing that we are not first on this list, but we are pleased to be among the nation’s highest performing large districts and I am confident our graduation rate is headed in the right direction,” Montgomery Superintendent Joshua P. Starr said in a statement.

Fairfax County had a graduation rate of 85 percent, based on Education Week’s calculations. Nationwide, the average graduation rate was 74.7 percent, a nearly 2 percentage point increase from 2009. Detroit public schools were at the bottom of the list at 46 percent.

Montgomery schools spokesman Dana Tofig said the system typically uses data that Maryland calculates to measure graduation rates in the district. He said state data show a slight increase in graduation rates. But he doesn’t discount the findings from Education Week, which uses data from the U.S. Department of Education to calculate its rates.

“The benefit of a report like this is it lets you see how you’re doing against other districts using the same metrics,” Tofig said. “We’re doing well, but you don’t like to see any calculation go down. It’s something we’ll look into in concert with other data that we have.”