By Gary Eason
Education editor, BBC News website
A school's truancy rate was 13 times greater than the figure reported in official performance tables.
The school had received a critical Ofsted report
The rate of unauthorised absence at St Thomas Aquinas RC High School in Manchester in 2003 is given in official statistics as 0.3% - very low.
But the rate is now said to have been 4%, several times the national average.
The revelation raises further concern about the reliability of the absence statistics - which are reported by schools themselves.
At the time school had been judged by Ofsted to be "failing" but is now much improved.
It has set up a special reward scheme for pupils who attend regularly.
Last year St Thomas Aquinas' truancy rate fell to 3.2% and it is expected to be lower this year.
Interviewed about the scheme by the BBC News website recently, head teacher John O'Callaghan said the background had been the 4% rate in 2003 - before he took over the school.
The national average in England's secondary schools that year was 1.1%.
But in the official Department for Education and Skills (DfES) performance data the school's absence rate was recorded as 0.3% - and reported as such in news organisations' tables.
It had also reported 0.3% in 2002, after 1.8% in the previous two years.
The watchdog, the Statistics Commission, said there needed to be "pretty rigorous validation" of such statistics.
Its chief executive, Richard Alldritt, said somebody should be looking at the figures and asking if they were credible.
But there was something of a grey area, with the Audit Commission considering the overall statistics but parent departments - education, in this case - responsible for the collection and reporting systems.
"Exactly who is responsible for auditing which bits is something we would be keen to see clarified," he said.
Similar issues were coming up in various departments, with systems designed for local purposes being used to produce national statistics.
Alerted to the St Thomas Aquinas anomaly some time ago, the DfES said simply that schools supplied its performance tables contractor with the figures, which were then checked - by the schools.
Despite repeated calls, no-one from the school has been available to comment on the discrepancy.
Manchester City Council assistant chief education officer Phil Hamilton said the "exceptionally low" 0.3% was "probably an error".
The more meaningful statistic for schools was overall absence, whether authorised or unauthorised.
"We would all acknowledge in every local education authority that there are some schools that feel pressurised by the publication of league tables to be inclined to authorise more freely than they might have done otherwise."
Schools were now being urged to "authorise ruthlessly", he said - in other words, not to approve a child's absence unless it was absolutely necessary.
The effect of changing the 2003 figure for St Thomas Aquinas would be to alter the average unauthorised absence for Manchester's mainstream secondary schools that year from 1.8% to 1.9%.
The authority would then drop a place in the national ranking, in which it was already 129th out of 150.
Last year it slipped further, with an average of 2.3%.
In a critical report on absenteeism earlier this month, the National Audit Office made mention of possible inaccuracies in the statistics.