As many as two in five teacher-assessed GCSE and A-level grades could be moderated downwards this year, Tes understands.
Insiders predict this summer's results will be a shock and fear a backlash because they could look "terrible".
Previous modelling by the FFT Education Datalab found that teacher-assessed GCSE grades were higher than 2019 scores, and that up to a third could be inflated.
Exam board modelling has backed up that level of disparity but insiders say it may well be higher still, with 40 per cent of grades being changed a "likely" outcome.
Insiders stress that despite the overall high number of grade changes, they may not change very much for each individual student.
However, the changes are likely to be bunched around the most common middle-ranking grades – such as 4 and 5 at GCSE – which, for many students, are also the most crucial.
The news comes as anticipation of the controversy likely to follow from next month's results builds.
Today, the Department for Education published a blog to counter concerns, raised in a petition signed by 180,000 people, that the government "had decided in their wisdom to reduce those [GCSE and A-level] grades by up to 33 per cent".
Concern over Ofqual approach
Meanwhile, the NAHT school leaders' union says it is concerned about the emphasis the statistical model used to standardise this summer's results will place on a school's historical performance.
It says this could further disadvantage students in schools in disadvantaged areas that are on ‘turnaround’ journeys.
Tes understands that exams regulator Ofqual is still working to finetune the standardisation model exam boards will use to calculate the necessary grade changes to keep results in line with previous years.
However, one senior exams source said the need to counter grade inflation in teacher assessed grades was unsurprising, given that teachers who knew pupils would wish to give them the benefit of the doubt.
Following the cancellation of this summer's exams, schools were asked to submit a “rank order” of pupils at each grade for each subject, as well as a teacher-assessed grade for each pupil on what it is thought they would have achieved had they sat the exam.
Exam boards will use Ofqual's standardisation model to ensure grades are in line at national and school subject level. This involves using historic subject achievement data for each school, which may be used to adjust grades where they appear to be inflated based on a school’s prior performance in that subject.
The regulator has previously said that schools’ historic performance in subjects would be given more weight where needed than “optimistic” teacher-assessed grades.
Fears for improving schools
NAHT assessment lead, Sarah Hannafin, said: "Teachers are experienced professionals who are absolutely in the best place to make a judgement on the grade their students would have got had they taken the exam – they have been provided with a lot of guidance to do this fairly.
"However, the grade awarded by teachers may not be the grade students receive on results day because exam boards will be applying a model of statistical standardisation.
"NAHT members have raised concerns about the emphasis placed on the historical evidence of centre performance. In particular, this could disadvantage schools on ‘turnaround’ journeys.
"These are, almost by their nature, schools serving deprived communities and their students could be further disadvantaged by Ofqual’s approach to the use of historical data to standardise grades. Applying a historical view on the school’s teacher assessed grades will likely see these grades reduced and those students not receive the improved outcomes they actually deserved.
"The most important outcome for 2020 is that students get the results they deserve and they should not be penalised for a centre’s past performance; this would be unfair."
Don't 'fixate' on grade changes
Geoff Barton, general secretary of the Association of School and College Leaders, said it was important not to "over-fixate on grades being moderated down".
“There was always likely to be some mismatch between the grades submitted by schools and colleges and the final grade awarded by the exam boards and this was recognised from the outset by the fact that centres were asked to place candidates in rank order to allow adjustments to take place.
“The reason for the mismatch is simply that schools and colleges will have given their best assessment of what the candidate would have received if they had sat an exam, and the exam board then applies a statistical model to make sure that grade distribution is similar to previous cohorts at a national level.
“This model doesn’t allow for the fact that in the normal course of events exam performance might have improved at a particular school or college.
'Difficult' results days
“It will be a difficult situation for schools and colleges to deal with on results days when they have pupils who they feel would have done better if they had sat an exam than they have done through this process.
“Everybody in the education system is aware of this situation and we are sure that sixth forms, colleges, and universities will all show a generosity of spirit over grades which are necessary for progression to further and higher education courses."
An Ofqual spokesperson said: “The exceptional arrangements in place this summer are the fairest way of giving as many students as possible the opportunity to move on in their lives, despite the cancellation of exams.
"In these circumstances, teachers are best placed to judge students’ likely performance if exams had gone ahead as planned. To bring consistency across different schools and colleges, exam boards will standardise grades, using a model developed with Ofqual.
"The model will work to ensure that national results are broadly in line with previous years. Any adjustments made as a result of standardisation will be precisely determined by exam boards for each subject in each school and college, and based on the evidence.
"Because of the speed at which these arrangements had to be put in place, it was not possible to provide national training to standardise judgements and therefore – as we have previously said - it is likely all centres will see some adjustments to their centre assessment grades to bring them into line with other schools and colleges.
"Such adjustments mean universities, colleges and employers can be confident this year’s results carry the same currency, and students can compete fairly for opportunities with students from previous and future years. We will publish final details of the standardisation model shortly.”
The Department for Education said: "This year, all exams in England were cancelled owing to the Covid-19 outbreak. We worked with Ofqual, the qualifications regulator, to develop a system whereby schools and colleges provide grades for students and then these grades are standardised by the exam boards to ensure national consistency and comparability to previous years.
"Thanks to this, young people can feel assured that the grades they receive this year will have the same currency as in any other year. Any students who do not feel the process has been followed correctly will have the right to appeal, and any who are not happy with their grades will have the opportunity to take exams in the autumn."