Appeals of GCSE and A-level grades will "take longer" than in normal years, Ofqual chair Ian Bauckham said today.
Speaking at the Confederation of School Trusts' (CST) conference, Mr Bauckham said there may be more evidence to review this year and that this will look different from one school centre to the next.
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He said: "While we have built in more time to deal with appeals this year by bringing forward results days, it is likely that appeals will take longer to process than in normal years.
"In normal years it is simply a matter of reviewing the marking of an exam script which is already well known to markers and is already on file with the board.
"This year, there may be several pieces of evidence to review, the evidence will look different from one centre to the next so would need to be submitted to the board when the appeal is made, so the whole process will take longer.
"We clearly cannot predict how many appeals there will be but boards are modelling for various scenarios."
GCSEs and A levels 2021: Appeals unlikely to be successful if schools have followed their grading policy
Mr Bauckham added: "While it will be possible for students to challenge if they believe the evidence range was unreasonable [in arriving at their teacher-assessed grade], a successful appeal will be unlikely if the centre has followed its approved policy by using a reasonable and consistent set of evidence for the whole cohort, permitted exceptions notwithstanding.
Schools can advise students if they were close to a higher grade
"Unlike in a normal examined year, there will be no marks or grade boundaries available as grades will be single, holistic judgements," he said.
"In considering whether to appeal, students will not know, for example, how strong a grade B or a 6 their result is. In other words, whether the evidence only just justified a grade 6 or easily justified a grade 6 and was getting close to a 7.
"Clearly, the school may be able to provide some advice on that but the decision on whether to proceed with the appeal will sit with the student."
Mr Bauckham also said that students should not be told their grades that are submitted to boards by their schools, but they may have a "reasonable" idea of their grade as they know what evidence it will be based on.
And he said that teachers must "set aside our feelings" when assigning grades and award them "dispassionately".
He said that grades must act as a "passport" this year for the next stage of education or training, and if they are awarded unfairly, this will devalue students' hard work or could mean that students progress on to courses they are not suited to.