An alliance of students, schools, local authorities and teaching unions in England has begun a legal challenge against the exam regulator's refusal to re-grade this summer's English GCSE papers.
A pre-action letter sent to Ofqual and exam bodies AQA and Edexcel threatens High Court action should they continue to refuse to re-mark the exams.
Some 180 pupils are part of the alliance that are complaining that GCSE English exams taken in June were not graded in line with the papers taken in January this year.
The group also includes 113 schools, 36 councils and 7 teaching bodies.
Their letter states: "It is inconceivable that two cohorts of students enrolled for the same course in the same academic year, who have undertaken the same work and invested the same effort, and who will be competing in future for the same opportunities, should be subjected to such radically different standards of assessment and award."
Jonathan Clarke, who sat his exam at St Matthew Academy in Blackheath, London, said: "I worked hard for a C grade in English and it has been taken away from me right at the end.
"That is just not fair. I am now having to redo the course with a different exam board over the next year at the same time as doing four A-levels."
The grade boundaries for the GCSE English foundation paper were changed for a C award by 10 marks between the January 2012 and June 2012 exams.
Schools and students say they relied on the published January grade boundaries in making their preparations for the summer exam.
Erica Pienaar, executive headteacher of the Leathersellers' Federation of Schools, said: "We understand that some grade adjustment goes on routinely, but schools are always informed about this and the adjustments are minor - one or two marks here or there.
"This year we were kept in the dark and at 10 marks the scale of the adjustment was huge."
Ofqual, which has seven days to respond to the alliance’s letter, has already conducted an inquiry into the problems but has stood by the grading.
The regulator found that January’s English papers were "graded generously" but the June boundaries were set properly.
Kevin Courtney, NUT deputy general secretary, told Sky News: "We have looked at the Ofqual inquiry and think it is weak and thin.
"The evidence we have is of children who have worked very hard who have got good grades in other subjects this summer and then who did not get the grade in English in a way that is completely surprising to their teachers."
Following the pre-action letter, an Ofqual spokeswoman said: "The matter is now in the hands of our lawyers and we will respond in due course."
The decision by ministers in Wales to re-grade pupils' English papers there has further ignited the row.