By Gary Eason
Education editor, BBC News website
One in six of the English secondary schools that improved their GCSE-level results since 2001 did worse in the core subjects of English and maths.
Schools are being asked to set targets for English and maths
A school's league table position is currently based on how many pupils get any five good GCSE passes.
The government is changing this from 2007, to include English and maths.
Unpublished results obtained by the BBC have been used to recalculate the tables so parents can compare schools on the old and new benchmarks.
New provisional figures for 2005, published on Thursday, show that nationally the average performance based on the new measure was 44.1%.
On the old measures it went up two percentage points this year to 55.7% - the biggest rise for a decade.
The 2005 results for each school will not be published until January.
But the 2004 figures obtained by the BBC show that of almost 2,000 mainstream state schools that had better GCSE-level results than in 2001, pupils' performance in English and maths worsened in one in six (17%).
The shift in fortunes arises because until now the government's GCSE-level attainment tables have included results from any GCSEs and equivalent vocational qualifications.
In particular, Intermediate GNVQs at any grade count as four, higher-grade GCSEs.
The national trend in GCSE-level performance
So pupils need only the equivalent of one other qualification to meet the existing benchmark of "five good GCSEs" - and in fact can do so without having any GCSEs at all.
But from next year, Education Secretary Ruth Kelly is imposing a new benchmark: two of the five must be actual GCSE qualifications at grade C or above in English and maths.
Schools Minister Jacqui Smith told BBC Radio 4's Today programme: "It is right that, as one form of accountability for schools, we know how well their pupils are achieving in their GCSEs and other qualifications.
"But I agree that needs to include English and maths and that is why we are going to be including those."
The move is part of the response to the government-commissioned Tomlinson report into 14 to 19 education in England, which pointed to a lack of "functional" literacy and numeracy.
In its 2005 tables, due to be published next January, it will be piloting this change with a view to implementing it in the 2006 tables (due out early in 2007).
Official guidance to schools sets out options for gaining English and maths qualifications - such as Key Skills instead of GCSEs - and says final decisions have not been taken on what will be used.
But the Department for Education and Skills insists this refers to additional information, and that the requirement for actual GCSEs is fundamental.
"All young people need a firm foundation in the basics - no matter what their choices are at GCSE - to ensure they have the skills needed to progress and succeed in further learning, employment and life - and that means English and maths," a spokesperson said.
To assess the likely impact, BBC Radio 4 documentary makers used the Freedom of Information Act to obtain the English and maths results for every school for each of the past four years.
The BBC News website has republished the current school tables - relating to the 2004 results - so the new measure can be seen alongside the old.
Ups and downs
Sixty-five schools scored 100% - every pupil achieved at least five good GCSEs including maths and English. On the old measure, 102 scored 100%.
Best overall, on the basis of the average points score of its pupils, is Wolverhampton Girls' High School, a grammar school in the West Midlands.
Thomas Telford city technology college in Shropshire, which headed the national GCSE-level table on the old measure, has 92% on the new - dropping more than 300 places.
Some well-known independent schools apparently do extremely badly. This is because they take International GCSEs which - unlike cake decorating certificates - do not count in the government's attainment statistics.
The best performing city academy is King's in Middlesbrough, which had 34% on the old measure and 26% on the new.
Some teachers feel the use of the GNVQ vocational qualification has helped to raise student performance significantly. Others think it has perpetrated a "con" on the public.
The general secretary of the Secondary Heads Association, John Dunford, said: "League tables only report what they are designed to measure and schools adapt their policies accordingly."
When they put more emphasis on English and mathematics, so would schools.
"The tables released today by the BBC should be no surprise. Schools enter students for exams that count in the league tables and that is, rightly or wrongly, how their success has been judged."
Mary Bousted of the Association of Teachers and Lecturers said: "This is yet more evidence if any were needed - that league tables are not serving anyone well. They are no help to schools, to teachers, to parents or children.
"The BBC's research confirms ATL's views that the league tables are pernicious."
Shadow schools minister Mark Hoban said: "Schools have been chasing government targets but at the cost of teaching our children English and maths."
He added: "The shine is starting to rub off Labour's claims of progress."
Liberal Democrat spokesman Ed Davey said the whole of Labour education policy was in question.
"This is the most damaging revelation that their approach to education has suffered in the last eight years," he said.
The Truth About School League Tables, Thursday 20 October, 20:00 BST on BBC Radio 4