Washington DC - US student test scores in reading and math remain below pre-pandemic levels as a worrying gap continues to widen between high and low performers, official data showed Wednesday.

The biennial tests of American fourth and eighth graders – correlating roughly to ages 9 and 13, respectively – showed improvements in 2024 for some students, but a steady decline for the lowest 10%.

"The most concerning pattern within our distribution is for our lowest performing students," Peggy Carr, head of the National Center for Education Statistics (NCES), told a press briefing.

She said that while the gap between high and low performers had further split across subjects and ages, eighth-grade math scores saw their widest difference since the assessment began.

The tests were administered in early 2024 to some 235,000 fourth graders and 230,000 eighth graders.

The last tests in 2022 sparked alarm, as they showed a significant across-the-board drop in scores from 2019 before the Covid-19 pandemic forced most US classes to move online.

The length of pandemic school closures quickly became a heated political debate, and ramifications on student performance continue to cause concern.

Data released Wednesday showed average fourth-grade math scores improved marginally in 2024, while the top 25% of performers had returned to 2019 levels.

Eighth graders, who would have been in elementary school during the pandemic closures, saw average math scores hold steady below 2019 levels – but while top performers increased marginally, the bottom 10% dropped significantly.

Reading scores fell for both eighth and fourth graders, the latter of whom would have begun school after the height of the pandemic.

"The continued declines since the pandemic suggest we're facing complex challenges that cannot be fully explained by the impact of COVID-19," said NCES associate commissioner Daniel McGrath in a statement.

The proportion of eighth graders failing a benchmark reading test was the highest since figures first were collected in 1992, while only one state out of 50, Louisiana, had better reading performance for primary school students than before the pandemic.

"I think it obviously comes to mind that we should be looking at what social media and the rise of the screen-based childhood is doing for reading habits and reading skills," Martin West, a member of the testing board and a Harvard education professor, told the press briefing.

The pandemic school closures prompted fierce political debate in the US, with Democrats generally more cautious in ending so-called remote learning, while Republicans sought to quickly return students to in-person classes.

Bill Cassidy, the top Republican on the Senate education committee, blasted the former administration of Democratic President Joe Biden and Vice President Kamala Harris for the falling scores.

"The most vulnerable children were hurt the most. This will be the failed legacy of the Biden-Harris education policy," he said in a statement.