WASHINGTON >> High school graduation rates for most states continue to improve, but Hawaii was one of five states that saw a decline, according to preliminary data released Monday by the Obama administration.

The U.S. Education Department says preliminary data indicate 36 states saw higher graduation rates for the 2013-2014 school year. The biggest gains were in Delaware, Alabama, Oregon, West Virginia and Illinois.

The nation’s overall graduation rate stands at 81 percent, an all-time high. The numbers show the District of Columbia with the lowest graduation rate at 61.4 percent and Iowa with the highest at 90.5 percent.

Five states had year-over-year declines: Alaska, Hawaii, Nevada, New Mexico and Oklahoma. The District of Columbia also slipped.

Federal data show Hawaii’s graduation rate dropped slightly to 81.8 percent in school year 2013-14 from 82.4 percent in 2012-13. State education officials noted that despite the dip, Hawaii remains above the national average.

“We will continue to work on strategies to ensure that students have the skills and knowledge to succeed in college and the workforce,” state Department of Education spokeswoman Donalyn Dela Cruz said in an email.

The federal data also show Hawaii’s graduation rate for special populations dipped to 59 percent from 61 percent for students with disabilities, to 77.6 percent from 78.2 percent for economically disadvantaged students and to 53 percent from 57 percent for students with limited English proficiency.

Eight states, the U.S. Department of Education said, showed no improvement over the previous school year: Colorado, Kansas, Maine, North Dakota, Ohio, Pennsylvania, South Dakota and Texas. Idaho did not have complete numbers to report.

Education Secretary Arne Duncan said the numbers were encouraging. “By all indications it looks like the nation will take another step in the right direction in terms of graduation rates,” Duncan told reporters Monday.

Final graduation rate data will be released next spring.