ATLANTA — New district-level test score data from the Educational Opportunity Project at Stanford shows students are performing worse in reading and math compared to a decade ago.

Researchers found reading scores were lower last year in 83% of school districts where data was available, while math scores were down in 70% of districts.

The report said the declines have impacted both wealthy and lower-income districts and crossed racial and geographic lines.

Education experts said there is no single reason behind the declines, but researchers noted test scores stopped increasing in the mid-2010s.

Researchers pointed to several possible factors, including reduced federal school accountability under No Child Left Behind, along with the rise of smartphones, social media and personalized school laptops.