Further warnings have been raised of potential "excessive grade inflation" following the publication of final plans for GCSE and A-level grading today.
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But experts have warned that the proposals lack detail over how grade inflation might be curbed.
David Laws, executive chairman of the Education Policy Institute (EPI), said: "Schools will be relieved at last to have the details on how they are expected to grade students this year, but we are still yet to see in the government’s plans how it intends to tackle the risk of excessive grade inflation.
"Without robust mechanisms in place which anchor the overall results at a level which is consistent with previous years, there is a danger that the value and credibility of this year’s grades are seriously undermined.
"The government has indicated that exam boards will apply checks and monitoring, but we also need a strong and risk-based quality assurance process which is able to scrutinise any results which look clearly out of line with previous years."
The warnings come after an Ofqual adviser said in January that exam boards would have difficulty restraining "wildly inflated" teacher-assessed GCSE and A-level grades this year, resulting in "Weimar Republic levels" of inflation.
Ofqual and the Department for Education say they do not expect high levels of inflation this year, because if schools and colleges submit unusually high grades to boards this would be picked up and investigated under external quality assurance.
But Natalie Perera, EPI's chief executive, warned that the proposals could result in "extremely high" levels of grade inflation.
Commenting on the government's plan for 2021 grades, Natalie Perera, chief executive of the Education Policy Institute (EPI), said:
"While the government was right to opt for teacher-assessed grades following the massive disruption to students’ learning, our concern is that significant risks remain with its approach set out today," she said.
"There is still a very high risk that we will see inconsistencies in the grades among different pupils and schools. Without timely and detailed guidance for schools on how this year’s grades should be benchmarked against previous years, and with classroom assessments only being optional, there is a significant risk that schools will take very different approaches to grading.
"This could result in large numbers of pupils appealing their grades this year or extremely high grade inflation, which could be of little value to colleges, universities, employers and young people themselves."