You can read the guidance here

The Government have said that they will ensure that GCSE, A and AS level students are awarded a grade which fairly reflects the work that they have put in. They have announced that there will also be an option to sit an exam early in the next academic year for students who wish to.

The Government have said that grades will be calculated using prior attainment data, 'other relevant data' and teacher judgement. In terms of the teacher judgement they have said the following:

"The exam boards will be asking teachers, who know their students well, to submit their judgement about the grade that they believe the student would have received if exams had gone ahead.
To produce this, teachers will take into account a range of evidence and data including performance on mock exams and non-exam assessment – clear guidance on how to do this fairly and robustly this will be provided to schools and colleges."

This is an early indication of the Government's plan for awarding grades and we will write to you with further information as we receive it. I would emphasise that, if teaching staff are to submit judgements about students’ progress, we will draw upon the quality of work up to and including the date the information is submitted to the exam boards. Therefore, it is necessary for students in years 10, 11, 12 and 13 to complete the work being set for them at home to the very best of their ability.

As soon as we hear more from the Government on this issue I will write to you again.