Calling it “a profound milestone,” Education Secretary Arne Duncan said Monday that the country has reached its highest graduation rate in history, with 80 percent of students receiving a diploma in 2012, the most recent year for which statistics are available.

“As a country, we owe a debt of gratitude to the teachers, students and families whose hard work has helped us reach an 80 percent high school graduation rate,” Duncan said at a gathering hosted by America’s Promise Alliance, a Washington-based nonprofit founded by former secretary of state Colin L. Powell. “But even as we celebrate this remarkable achievement, our students have limitless potential, and we owe it to all of our children to work together so they all can achieve at higher levels.”

Nationally, girls had a higher graduation rate, at 84 percent, while boys had a rate of 77 percent.

The data, collected by the National Center for Education Statistics, estimate the graduation rate by dividing the number of high school graduates in a class by the number of students who entered that class as freshmen four years earlier, with some adjustments made for transfers.

Despite the high-water mark of 80 percent in the class of 2012, disparities persist.

In many states, one-third of students from low-income families did not graduate. Black students had a 69 percent graduation rate and Hispanic students had a 73 percent rate, while 86 percent of white students and 88 percent of Asian students earned high school diplomas. English-language learners and special-education students had below-

average rates of 59 and 61 percent, respectively.

And graduation rates varied from state to state. For example, while 79 percent of students with disabilities graduated from high schools in Arkansas in 2012, just 33 percent did so in Louisiana. While 83 percent of English-language learners graduated on time in West Virginia, 24 percent graduated in Arizona.

Locally, Maryland and Virginia were above average at 84 percent and 83 percent, respectively. The District of Columbia had a graduation rate below the national average, at 59 percent.

It is not unusual for major cities to experience a higher dropout rate and lower graduation rate than states. One study found the graduation rate for the class of 2005 in the nation’s 50 largest cities was 53 percent, compared with 71 percent in the suburbs.

High school graduation rates are one measure of school success, and educators and policymakers have been trying for decades to decrease the number of high school dropouts.

Official statistics had long overstated the nation’s high school graduation rates, with the Education Department putting the average above 80 percent and some states reporting rates above 90 percent. States used dozens of different reporting methods, with some figuring into their rates those dropouts who later earned the equivalency certificate known as a General Educational Development diploma, or GED.

In 2005, the Education Department began publishing an official estimate of graduation rates, and all 50 states agreed to adopt the same method of calculating those rates by 2013.

The country needs to focus on the students nationwide who don’t graduate from high school, Duncan said.

“Let’s talk in concrete terms about who is behind those numbers,” Duncan said. “That 20 percent represents 718,000 young people, among them a sharply disproportionate share of African Americans, Hispanics, Native Americans” as well as special-needs students and English-

language learners, he said.

Those students without high school diplomas face a bleak life of “poverty and misery,” Duncan said.

“High school graduation may have once been a finish line, but today it is just a beginning,” Duncan said, adding that the nation needs to also concentrate on college graduation rates, which have been slipping.

A generation ago, the United States led the world in college graduation rates. But today, 11 other countries have surpassed the United States, where 43 percent of young people have a college degree, Duncan said.