The British government faced criticism on Thursday after education officials downgraded more than a third of pupils' final grades in a system devised after the coronavirus pandemic led to cancelled exams.
Although the newly released results for 18-year-olds showed record-high grades and more students accepted to university courses, exam boards downgraded nearly 40 percent of pupils' grades in England.
Pupils across Britain were unable to sit summer exams as planned due to the COVID-19 lockdown and instead have received a moderated grade based on an assessment by their school or teacher.
But the system, brought in to ensure the marks were not significantly higher than in previous years, could now see thousands fail to get predicted grades and miss out on university offers.
"While there has been an overall increase in top grades, we are very concerned that this disguises a great deal of volatility among the results at school and student level," Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said.
"We have received heartbreaking feedback from school leaders about grades being pulled down in a way that they feel to be utterly unfair and unfathomable.
"We are now calling on the government and the exam regulator Ofqual to review the situation as a matter of urgency -- and we would warn them against simply digging in their heels and insisting all is well."
In Scotland, which published its results last week, the devolved government on Tuesday said it would scrap the moderation process after complaints that it had hit poorest pupils' marks hardest.
- 'Urgently rethink' -
Education Secretary Gavin Williamson, who is responsible for schooling in England only, has ruled out further changes to the grading system there despite the widely anticipated backlash.
Ahead of the results he announced pupils would be able to use mock exam results as the basis for an appeal and that they could also choose to sit exams in the autumn instead.
"This is the right approach to go forward," he insisted Thursday.
But critics have noted there is no standardised system for mock exams and there remains a lack of clarity over the appeals process.
"Something has obviously gone horribly wrong with this year's exam results," Keir Starmer, leader of the main opposition Labour Party, said.
"Parents, teachers and young people are rightly upset, frustrated and angry about this injustice.
"The system has fundamentally failed them," he added, urging the government to "urgently rethink" the system.
Overall, the number of pupils in England, Wales and Northern Ireland awarded the highest A* grade jumped to nine percent, which is the highest proportion since the top grade was introduced in 2010.
The figures, published by the Joint Council for Qualifications and impacting around 300,000 students, also showed nearly 28 percent were awarded either an A or A*, which is up by over two percent on last year.
Meanwhile, UK university officials said 358,860 students had so far been accepted on courses, up nearly three percent on 2019.