Headteachers have told parents that the tables, containing school-by- school results of tests taken by 11-year-olds last summer, are "misleading".
Among the details parents are being asked to take into account are absences of children eligible to take the English, maths and science tests, but whose marks count as zero when the tables are calculated.
Heads have sent the letters on the advice of the National Association of Head Teachers (NAHT), which last January lost a High Court challenge to aspects of the league tables.
Their action is the second attack on the tables in a week. On Wednesday, another teaching union, the Association of Teachers and Lecturers, published researchwhich suggested almost a quarter of children had been given the wrong grade in English.
Gillian Shephard, Secretary of State for Education and Employment, yesterday denied the tables were misleading. Both unions were "seeking to discredit the biggest information exercise of its kind for parents and the wider public ever undertaken"., she said.
Mrs Shephard pointed out that Nigel de Gruchy, general secretary of the National Association of Schoolmasters Union of Women Teachers, had blamed attacks on the system on a "tiny clique".
The tables are being published on Tuesday only after a about-turn a year ago by Mrs Shephard,. She originally had said she would wait a further year until the tests for 11-year-olds had "bedded down".
Although the Department of Education and Employment will publish the data in alphabetical order, it will be turned into tables by newspapers.
NAHT general secretary David Hart yesterday said the union wanted parents to have a letter from school putting the league tables in context before publication day.
An absence of only four children from a group of 17 in one primary eligible to take the tests had meant a distortion of almost 25 per cent, making the school's performance appear far worse than it was, he said.
The NAHT is concerned that by dropping only a few extra marks, pupils can slip from level four, which is the standard fixed by the Government as appropriate for 11-year-olds, to level three, making it appear they are up to two years behind. Schools also object to the fact that no account is taken in the tables of the number of children with special educational needs who take the tests and may achieve lower scores than their classmates, dragging down their school's league table placing.
Mr Hart denied his union was using "spoiling tactics" in advance of the tables' publication. He said: "There is an obligation on the Government to make sure the only thing they publish is accurate and fair. We contend that it will not be accurate, it will not be fair and it will substantially depress results in a number of schools."
The NAHT was not against testing but opposed league tables in the present form, he added. Mr Hart said whoever became Secretary of State for Education after the election ought to examine different ways to represent children's performance.Reuse content