In an era of high-stakes testing brought on by the No Child Left Behind Act, new research finds the overall quality of reading and math instruction to be in decline, and the students most in need of high-quality teaching as the least likely to receive it.

Linda Valli, the University of Maryland education professor who collected the data, said her findings cast doubt on the effectiveness of NCLB, the federal legislation signed in 2001 aimed at raising standards and closing the “achievement gap” between high-performing and low-performing students.