US charter school students are lagging six months behind pupils in state schools, according to data that the Bush administration is accused of trying to hush up.
The findings pose awkward questions for the White House, which extols charter schools as replacements for ailing state schools; and officials are scrambling to explain why they have sat on them since last November.
Just one in four charter nine to 10-year-olds are proficient at reading or maths, according to results from the 2003 National Assessment of Educational Progress (NAEP), America's only nationally-administered exams.
This contrasts with 30 per cent in reading and 32 per cent in maths among peers at comparable state schools. Overall, the test scores equated to a half-year knowledge gap, said the American Federation of Teachers, the US's second largest teachers' union, which culled the charter scores from unprocessed official data.
The data offers the first US-wide comparison between charters and education authority-run schools.
"This is part of a bigger research picture that finds little evidence that charters are performing better than public schools," said Alan Krueger, professor of economics and public policy at Princeton University.
"What's surprising is that the Education Department has not released the data in more conventional format."
The information was supposed to have been collated in January, two months after other NAEP reports, but was delayed while officials performed a "sophisticated analysis" to qualify the data, said Sharif Shakrani, deputy executive director of the National Assessment Governing Board, which sets the exams.
It "should have been put into the public domain", said Shakrani, who called the union's action "completely appropriate".
Responding to last week's revelations, US education secretary Rod Paige called for a distinction between "students falling behind and students climbing out of the hole in which they found themselves". He said the purpose of charters is to provide an alternative to students poorly served by their previous schools.
But Amy Wells, education professor at Columbia University's Teachers College, said there was no evidence that charters were predominantly dealing with legacy problems from state schools. "Some have been open for 10 years. To blame state schools 13 years into the charter movement is ludicrous," she said.
"Neo-liberal, free-market ideology is fuelling charters. The idea is to deregulate and let the market work its magic, but this report says this doesn't work in education."
Roughly 2,700 of America's 88,000 state schools are charters and their ranks are expected to swell as the White House's sweeping No Child Left Behind Act tags thousands of schools for closure for missing testing targets. President Bush has apportioned $318m (pound;175m) for new charters in 2005.
Charters are operated autonomously by businesses, teachers, churches and other groups as another public education option. But the sector has been plagued by profiteering scandals. Last week, California's largest charter operator - under investigation for alleged financial impropriety - closed more than 60 schools, leaving 10,000 students school-less.