When Rashaan Payne was 2 years old, his pediatrician noticed that he was not talking at the level of most children his age. After autism was diagnosed, Rashaan began receiving speech therapy once a week at his home on the South Side of Chicago, paid for by the federal and state governments.

When he turned 3 in October, federal law mandated that he leave that program and be evaluated for services within the Chicago Public Schools. But while his mother, Treva Thompson, said she has filed paperwork and repeatedly called the neighborhood school, Rashaan has yet to be evaluated. She is worried that after making progress, her son will lose ground.

“It’s like he’s at a pause now,” said Ms. Thompson, who left her factory job packing ice cream cones to stay home and take care of Rashaan. “When you’re dealing with special-needs children, you need to be consistent with whatever you’re doing. You can’t do something and then stop in the middle of it.”

In a complaint filed on Monday with the Illinois State Board of Education, a nonprofit advocacy group says that thousands of children are in Rashaan’s position because the Chicago Public Schools have repeatedly failed to evaluate children with disabilities and move them into special education preschool programs.