Under federal law, students with disabilities are supposed to attend class with their typically-developing peers as much as possible, but new research suggests that may not be happening.

In what researchers say is the first study to look at national trends in school placement for students with intellectual disabilities over the last four decades, they found that the majority of these children spent most or all of their time in self-contained settings.

What’s more, while inclusion rates rose during some periods in the nearly-40-year span, progress seems to have halted in recent years, according to findings that are set to be published in the American Journal on Intellectual and Developmental Disabilities.

Under the law now known as the Individuals with Disabilities Education Act, students should be placed in the “least restrictive environment.”

“Given the legal mandate, it is surprising that such a large proportion of students are consistently placed in restrictive settings,” said Matthew Brock, an assistant professor of special education at The Ohio State University who worked on the study.

Researchers relied on several data sources to examine placement trends for students with intellectual disabilities ages 6 to 21 across the country between 1976 and 2014. Over the decades, they found that between 55 and 73 percent of students in this population spent most or all of their day in segregated placements.

Starting in 1990, researchers found that an increasing proportion of students were educated in less restrictive settings, but that progress plateaued in recent years.

By 2010, the number of students with intellectual disabilities spending at least 80 percent of their school day in general education classes peaked at 18 percent. But that figure slid to 17 percent by 2014, the study found.

Brock noted that variations in student placements across states suggest that opportunities remain for greater inclusion.

“I don’t want to send the message that all kids with intellectual disabilities should spend 100 percent of their time in general education classrooms,” he said. “But I think we need to find opportunities for all kids to spend some time with peers who don’t have disabilities if we are going to follow the spirit and letter of the law.”