The High Court will rule on an unprecedented legal challenge over GCSE exam grades.

An alliance of hundreds of pupils and schools and scores of local councils, as well as teaching unions, are seeking judicial review.

They are accusing the AQA and Edexcel exam boards of unfairly pushing up the grade boundaries for English last summer in what amounted to "illegitimate grade manipulation" and "a statistical fix" involving exams regulator Ofqual. Both the boards and Ofqual deny acting unfairly or unlawfully.

Lord Justice Elias and Mrs Justice Sharp will give their judgment at the High Court in London. They were told at a hearing in December that an estimated 10,000 pupils who sat exams in June last year missed out on a C grade - the minimum grade normally needed to go into further education.

Clive Sheldon QC, appearing for the alliance, said the lower grades were not the fault of the students, who had "worked well and hard". He said the evidence of unfairness was overwhelming.

Ofqual had given an instruction to avoid "grade inflation". Bodies awarding grades were required to meet tolerances based on statistical predictions derived from students' performances in Key Stage 2 examinations five years previously.

The predictions, which were used as a "straitjacket" rather than as a guide, were that too many students were going to get a C grade or better in GCSE English, said Mr Sheldon. A decision was taken to push up grade boundaries for the exams marked in June to bring down the numbers of good grades for the year as a whole.

Mr Sheldon said that had resulted in students being unjustifiably "clobbered" to meet the Ofqual requirement of no apparent grade inflation. He argued the "gross manipulation" that took place amounted to "conspicuous unfairness and an abuse of power".

Schools, teachers and students had reasonably expected that those sitting the June examinations would be treated consistently with students who had earlier taken papers and submitted controlled assessments, argued Mr Sheldon.

He said the June students should be put in the position they would have been in had the grade boundaries in January been applied to them.