Nevertheless, while 69 per cent of nine-year-olds had problems with maths, US education leaders pointed to steady progress since 2000, when 78 per cent were found wanting, as the first fruit of President Bush's standards-based reforms.
"These results show the education revolution has begun," said education secretary Rod Paige.
Stagnant reading performance was less encouraging, however, with only 32 per cent of pupils demonstrating proficiency in reading across both age brackets - barely improved on 29 per cent 10 years ago, and one point down on last year.
Moreover, while minority students notched steady improvement, they still trailed white pupils by a wide margin. Just 13 per cent of black students were proficient readers compared with 41 per cent of whites.
The National Assessment of Educational Progress (NAEP) represents the broadest snapshot of academic standards across America, where tests are otherwise administered on a state-by-state basis. This year's tests spanned a sample of 686,000 students attending 13,600 schools across all 50 states.
The Bush administration is setting new store by NAEP in its quest to establish national education standards.