Lawyers challenging this summer's GCSE English grades have accused two exam boards and the watchdog Ofqual of grade manipulation and a statistical fix.
An alliance of pupils, teaching unions, schools and councils is seeking a judicial review of last-minute shifts in grade boundaries in June's exams.
The hearing is expected to last up to three days with a verdict likely before Christmas.
Ofqual and exam boards AQA and Edexcel will give evidence later this week.
Clive Sheldon QC, representing the claimants, said there was evidence of "manifest unfairness by AQA and Edexcel, procured and approved and certainly not corrected by Ofqual".
He added that the raising of the mark needed to get a C grade between the January and June exam units left thousands of students victim to "inconsistency within the year group" and "a lack of even-handedness".
Mr Sheldon told the High Court in London that in one AQA exam unit the mark needed to achieve a C rose from 43 to 53 marks out of 80 from January to June although there was no change in the difficulty of the paper.
The claimants are calling for GCSE English exam papers taken by pupils last June to be regraded in line with grade boundaries used for the same exams in January 2012.
The alliance says some 10,000 pupils consequently missed out on a C grade in GCSE English, which is a crucial benchmark used for entry into further education, vocational training and employment, but the decision affected all the grade margins.
In total up to 50,000 pupils in England, Wales and Northern Ireland could receive higher grades if the judges uphold the challenge, says the alliance.
Some 2,300 students who took exams set by the Welsh exam board WJEC in Wales have already been regraded on the orders of the Welsh government, which regulates exams set there.
The case has been brought by a total of 167 individual pupils, supported by 150 schools and 42 councils, plus six professional bodies, including teaching unions.
They will say that between January and June 2012 it was decided that too many students were going to get a C grade or better in GCSE English, so 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.
They argue that this amounts to a crude "statistical fix" which was unfair and against "natural justice".
"Now that we have looked at all the evidence and made final preparations for the court case, we remain certain that it is the right thing to do," said Brian Lightman of the Association of School and College Leaders (ASCL).
"Thousands of young people in England were unfairly downgraded in June in order to compensate for mistakes made earlier in the year.
"The only fair course of action for these students is to regrade the papers. While many of them will have moved on to college courses or to other options, their grades will remain with them for years to come and that is unjust."
Ofqual and the two exam boards have said they will not be commenting on the proceedings until they are over, but Ofqual has vowed to "rigorously defend" its decisions. In its final report into the grading row, Ofqual insisted the June boundaries were correct. It also suggested the grade boundaries had had to be moved in response to teachers "significantly" overmarking controlled assessment papers.
Some 45,000 pupils resat GCSE English last month, with the results expected next week.