Kevin Maxwell, right, the Prince George’s County school system’s chief executive officer, lauded the county’s improvement in its high school graduation rate. (Mark Gail/For The Washington Post)

Maryland’s high school graduation rate hit a new high for the Class of 2015, with students in Prince George’s County making some of the state’s greatest gains and reaching a record level, according to newly released numbers.

Statewide, 87 percent of students earned their high school diplomas in four years, a slight uptick over 2014 and five percentage points better than in 2010.

In Prince George’s, rates rose to 78.8 percent, from 76.6 a year earlier, a one-year jump of more than two points that comes as the school system has been pushing hard to boost student achievement.

State data showed the rate in Prince George’s at its highest since 2010, when Maryland changed the way it makes such calculations. The state now follows a cohort of students from ninth grade through high school, a method officials describe as more accurate.

“This is tremendous improvement, and I am very proud of our students, staff, teachers and administrators for their hard work in achieving another impressive milestone,” Kevin Maxwell, chief executive officer of Prince George’s County schools, said in a statement.

Maryland officials said the increase for Prince George’s stood out among the state’s 24 school systems, second only to Somerset County, a far smaller school system on Maryland’s Eastern Shore.

Monique Davis, deputy superintendent in Prince George’s, noted that the yearly increase is the second in a row and credited changes in the district’s culture, a “laser-like” focus on data and stronger monitoring of student and staff performance.

The district also continues to use an early-warning system to target middle school and high school students who need intervention and support. Ten high schools improved by three points or more, Davis said.

On Friday, the school system delivered a presentation on its successes to the Maryland State Board of Education. “It was absolutely wonderful to have the principals share their stories,” Davis said.

In Montgomery County, the state’s largest school system, the four-year graduation rate was fairly flat, with a slight dip from 89.7 percent to 89.4 percent.

Across Maryland, there were major gaps by race and ethnicity. The graduation rate statewide was 96 percent for Asian students, 92 percent for white students, 82 percent for black students and 77 percent for Hispanic students.

Over a five-year period, the state rate for African American students was up by more than eight percentage points, while the rate for Hispanic students rose more than three points.

“Each student who graduates from high school is a success story, and those students, families, and schools should celebrate that success,” said Jack R. Smith, the state’s interim superintendent of schools, in a statement Friday.

Smith, recently named superintendent in Montgomery County — where he starts July 1 — described a high school diploma as a first step. “We continue to strengthen our standards to better prepare each student for life beyond high school — be it further education, the workforce or both,” he said.