Charter schools are enrolling a smaller portion of special-education students than traditional public schools, The Wall Street Journal reports, citing a congressional study released Tuesday.

The Government Accountability Office report said students with Down syndrome, attention-deficit disorder, and other learning disabilities made up 8.2 percent of students at charter schools in 2009-10, compared with 11.2 percent for public schools. Federal law requires that all government-funded schools accept almost any disabled student seeking to enroll.

Teachers’ unions and other critics have claimed that charters — which are taxpayer-funded but run by private entities, many of them nonprofit — limit enrollment by special-ed students to save money and achieve higher standardized-test scores. Charter advocates say their numbers are lower because the parents of disabled children tend to choose public schools, which have more experience in special education.

The study said it was unclear what caused the gap, but it cited “anecdotal accounts” suggesting that some charter schools discourage special-needs students from enrolling and deny placement to the most severely disabled. It also noted that traditional schools are generally larger than charters and have more special-ed resources.