The number of public school students in Oregon considered homeless has increased for the third year in a row, according to an annual report released by the state Department of Education.

More than 21,000 students in Oregon public schools are homeless — nearly 4 percent of the student population.

The vast majority of students who qualify as homeless do have some sort of shelter at night, most often shared or temporary housing arrangements that aren’t considered stable.

But just because they’re not sleeping outside doesn’t mean there’s not an impact, said Marti Heard, the homeless program liaison for Portland Public Schools.

“If you’re staying someplace where the utilities (are) off, you may not be warm enough, you may not have light for studying,” said Heard. “These students are really at risk for falling behind academically.”

The Oregon Department of Education report blames a lack of affordable housing statewide for the increasing number of homeless children.

“We know that students dealing with difficult life circumstances have a much harder time in the classroom,” Deputy Superintendent Salam Noor said in a statement. “Our goal is to make the school environment as stable as possible for homeless students.”

The percentage of homeless students reached double digits in 18 Oregon school districts this year. While those districts tended to be in rural areas where a small fluctuation can dramatically shift the percentage, a handful of larger districts had double-digit percentages of homeless students, including Lincoln County SD, Central Point SD in Jackson County, and Sweet Home SD in Linn County.

The numbers included in the new report reflect the student homeless population in the 2015-16 academic year.