In any normal year, a sixth-form teacher would be pleased to be handed three four-page, well-researched essays by an exam candidate. But as schools broke up for Easter, one head of history in Kent had to tell his conscientious student she was too late.

Only work submitted before 20 March – the date schools closed because of the coronavirus, with this year’s public examinations cancelled – can be counted towards students’ final grades. Even practical coursework for arts subjects cannot be completed.

“She was upset because she had not done any timed essays in the year that were long and precise enough. She thought she would get 60 and not the 75 she needed for an A*,” he said.

“But the rule is fair because some students have much better resources to work at home than others, and there would be too much scope for cheating,” added the teacher, who has been told by his senior leadership team not to comment publicly on plans to grade students.

Teachers tell of students in tears because the cancellation of exams means they have lost the chance to raise their grades with Easter revision.

Fortunately, say assessment experts, students need not be downhearted. They say this year’s results will be based on teachers’ recommendations, called “centre assessment grades”, plus statistical moderation by examiners, who can change the grades up or down. This combination, they say, is likely to be fairer than the usual exam system.

“This done well by teachers with integrity is the fairest method, and beats the reliability of the current exam system hands down,” says Dennis Sherwood, who monitors assessment systems and has worked as a consultant at Ofqual, the exams regulator.

A better exam system may be a welcome legacy of the coronavirus pandemic, he believes. “If we can demonstrate that teacher assessment works, is trusted, and can be done a lot more quickly and be less costly with a lot less emotional wear and tear for students, we are in a much better position in coming years to evolve something better than the present system.”

He says Ofqual’s own research, published in November 2018, shows that about 200,000 – a quarter – of A-level grades would be changed if the scripts were re-marked by a senior examiner. For GCSEs, 1.3m grades did not match, again one in four.

While it may shock the public to realise just how unreliable grades can be, it is no surprise to most teachers. “I am aware every year, particularly with my subject, psychology, that the outcomes vary to a degree I have never been able to predict,” says the head of social sciences at a large academy school in the south of England.

“Exams themselves do not provide a highly reliable outcome. Nor are results over the years necessarily stable. One year we had 80% A or A* for A-level, and the next year it went down to 20%. It comes down to the luck of which questions come up on the paper and how the scripts are graded.

“For a lot of us, grades are linked to our sense of wellbeing and efficacy. Ten years ago I felt they were my grades as well, but I don’t feel that any more, I have learned to accept that I cannot control the outcomes. All I can do is teach as well as I can.

“The problem is that this year I have been asked to put in the grades myself. I’ve been talking to other people in the same boat and we don’t like it, because our job is to be alongside the student, helping them to do as well as possible,” he says.

Usually exam papers are marked by examiners and then, once the overall picture emerges, the different exam boards decide the number of marks needed for each subject grade. They work out how many candidates will get each grade and submit the estimates to Ofqual. The regulator then collates the data from all the boards to check that the proportion of, say, A grades awarded nationally is in line with previous years and fits with the information it holds about the prior attainment of the cohort.

This year teachers will recommend what grades students should be awarded and put students in rank order within the grades – the ones most likely to achieve that grade at the top, and the weakest near the grade boundary at the bottom. To deliver the fairest outcomes, Sherwood suggests students should be ranked first, and then the grade boundaries determined to reflect the school’s previous results in the qualification.

Ofqual tells teachers they should take a student’s prior performance over the course into account, but suggests results in previous examinations should be used “at centre level” to judge the ability of the class and year group, not the individual candidate. In other words, an A-level student should not be marked down for their GCSE results.

A student working at an A grade during the year and hoping to raise it to an A* by the exam date, may still get the coveted top grade if the school got five A* grades last year and thinks he or she is one of their top five students.

Teachers have been told not to tell students what grades they are submitting, because they may not be the final ones. It could leave teachers open to bribery, abuse or even legal action. Ofqual has yet to announce how students will be able to appeal grades they feel are unfair.

A head of English, who wants to remain anonymous, says his department will not be relying much on teacher opinions, but on aggregated data showing how well the candidates performed in mocks, tests and assignments compared to each other.

“Results are capped by the proportion of grades awarded in previous years, so if you got five A*s last year, that’s what Ofqual will expect to see. It means that you have to be very careful, if you move someone up on a hunch they would have done better in the actual examination, you are moving someone else down,” he says.

Teachers also should be aware that research has consistently demonstrated unconscious teacher bias against some groups based on race, class, gender and even personal qualities such as attractiveness, he explains.

There will be anomalies for some schools. Putney High, an independent school in London, for example, last year added Fashion and Textiles to its art and textile A-levels; only six girls chose it, with half getting an A* grade and half a B, making an A grade average. This year, with numbers doubling, the school expects two-thirds to get an A* and one third an A.

“This year’s is a very strong cohort and they deserve the grades but will the exam board look at last year’s results, think we have been too generous, and put their grades down? That would be very unfair,” says Stuart McLaughlin, the school’s head of textiles.

Fast-improving schools could also miss out if their grades are mapped to those awarded before, as could those where the intake changes, such as a boys’ school admitting girls to the sixth form. But a spokesman for Ofqual said it was still consulting on its model of “standardisation” that could take such circumstances into account.