WASHINGTON -- The nation's high-school graduation rate has ticked up slightly to 82 percent, a new high.

The Education Department said Tuesday the rate for the 2013-14 school year -- up from 81 percent the previous year -- was the highest since it started using a new, uniform measure in 2010. Still, the numbers show nearly 1 in 5 students leaving high school without a diploma.

Education Secretary Arne Duncan said too many students still aren't graduating, but he praised the numbers as encouraging.

"America's students have achieved another record milestone by improving graduation rates for a fourth year," Duncan said in a statement. "We can take pride as a nation in knowing that we're seeing promising gains, including for students of color."

The data showed black and Hispanic students made some progress in closing the achievement gap with their white counterparts. About 72 percent of black students and 76 percent of Hispanic students earned diplomas in 2013-2014. For white students, the rate was 87 percent.

English language learners and students with disabilities had the lowest graduation rates, at 62 percent and 63 percent, respectively.

The latest numbers, though, drew concern from groups advocating for a 90 percent graduation rate in the coming years. To reach that mark, the rate needed to increase each year by at least 1.3 percent -- something it missed for 2013-2014.

"For the first time in four years, the country is not on track to reach the national goal of a 90 percent on-time high school graduation rate by 2020, missing by just a few tenths of a percent," said a statement from the four organizations leading the GradNation campaign -- the Alliance for Excellent Education, America's Promise Alliance, Civic Enterprises and the Everyone Graduates Center at Johns Hopkins University's School of Education. "The last leg of this campaign will be very challenging, and ... we must redouble efforts to reach our goal."

Across the country, Iowa had the highest graduation rate, at 90 percent. The District of Columbia had the lowest, with a 61 percent graduation rate.

The graduation rate is calculated by using a formula in which the number of graduates in a given year is divided by the number of students who enrolled four years earlier. In 2008, the Bush administration ordered all states to begin using this method, moving to a calculation that required them to track each student individually, giving a more accurate count of how many actually finish high school.