Why is Texas last in America – by far – in the percentage of its school children in special education? Brian M. Rosenthal, an investigative reporter in the Houston Chronicle's Austin bureau, asked himself the simple question in early 2016 as he delved into data about the plight of kids in Texas.

There had to be some sort of policy or practical explanation, since mental and physical impairments are biologically determined at rates that shouldn't differ all that much between Texas and Tennessee, Minnesota and Maine. For some reason, only 8.5 percent of students in Texas receive the kind of specialized instruction mandated by federal law for all kids with disabilities. In Tennessee, its 13.1 percent – right at the national average – while Minnesota has 14.6 percent of its kids in special ed and Maine – the highest in the nation – has 17.5 percent.

Burrowing into thousands of pages of records and interviewing over 300 parents, teachers, school administrators and education experts, Rosenthal discovered that the Texas Education Agency had quietly set forth a "benchmark" that directs all 1,200 Texas school systems to limit special education enrollments to 8.5 percent – which, perhaps not surprisingly, became the statewide average in 2015.

It seemed remarkable that the state could essentially impose a de facto cap on special education enrollments in the state 12 years ago without anyone in the public – or the U.S. Department of Education, for that matter – realizing precisely what had happened. But Rosenthal found that many advocates, and even more parents, had no idea that their long, frustrating and often unsuccessful quests to get services for their children led back to this 8.5 percent limit.

The TEA required many school districts above 8.5 percent to file "Corrective Action Plans" describing the steps they would take to get below the magic number. Some required multiple rounds of meetings before an evaluation could take place. Others offered disabled kids cheaper alternatives called Section 504 plans that provided accommodations, such as a seat in the front of the class, but no additional instruction. Still others removed clearly disabled children from special education programs and returned them to regular classrooms.

Denied, the investigative series based on Rosenthal's six-month investigation, has already triggered a review by the Department of Education in Washington and almost surely will be the subject of hearings during next year's legislative session, with both Democratic and Republican lawmakers calling for an end to the 8.5 percent target.

Brian M. Rosenthal is a Houston Chronicle reporter based in our Austin bureau. He can be reached by e-mail at Brian.Rosenthal@chron.com or by Twitter: @brianmrosenthal