The Ofqual report commissioned to investigate the controversy surrounding the high fail rate in this year's GCSE results has found the January exams were "graded generously".
The exam regulator concluded the overall subject grades awarded were "correct" and said the grade boundaries in June were higher than in January because exam boards were "better equipped to make judgements".
Ofqual said exam boards have agreed to offer students who completed the new GCSE English qualification in June an extra opportunity to take an early re-sit, after "recognising the strength of feeling".
Chief Ofqual regulator Glenys Stacey said: "People were particularly concerned about the June grade boundaries.
"We have found that examiners acted properly, and set the boundaries using their best professional judgement, taking into account all of the evidence available to them. The June boundaries have been properly set, and candidates’ work properly graded.
"The issue is not the June, but the January boundaries. Again, examiners used their best judgement in setting these boundaries, but they had less data and information to work with.
"Most candidates were not sitting at the time, they were waiting for June, and because they were new qualifications, examiners could not rely so much on direct comparisons with the past. As a result, those grade boundaries were set generously."
Stacey added Ofqual had thought "carefully" about what should be done and reiterated the watchdog's job is to maintain standards over time so grades awarded are comparable from year to year.
The report looked at the evidence and concerns of schools and colleges and worked closely with exam boards. Stacey said the boards had been "very responsive".
"Recognising the strength of feeling, they will be offering early resits for students who sat the June units.
"We will talk with schools, exam boards and assessment experts to see what lessons can be learnt and what can be done better in the future."
Kevin Courtney, deputy general secretary of the National Union of Teachers, called the report "simply scandalous".
"This is a very weak and disappointing report and we reiterate the fact that there needs to be an independent inquiry. This report does not recognise that they have let down teachers, students and schools.
"It still remains the fact that it is simply scandalous to change the grade boundaries halfway through a school year. This scandal constitutes a real attack on these students’ chances to continue in education, employment or apprenticeships."
For the first time in history, GCSE grades have fallen and many students found themselves under-achieving. The high fail rate prompted an outcry from students and teachers alike, with heads calling for an inquiry.
Two students - Jodie Sullivan and Lade Ajose - have even started a petition to have their GCSE exams re-marked.
Jodie, 16, told The Huffington Post UK: : "We're obviously very disappointed that Ofqual have decided not to look again at the exam marks. It seems completely ridiculous to me that they can mess around with grades that are so important to peoples' futures then refuse to re-mark them when it gets out in the open."
Lade, also 16, added: "We're not sure whether there's any chance of them changing their mind. I've seen that some people are talking about legal proceedings. I just want my papers and my friends papers to me looked at again. It's stupid that they won't be.""
Earlier on Friday it emerged GCSE grading problems were identified three years ago but Ofqual failed to act on the advice, according to the Times Educational Supplement.
ASCL general secretary Brian Lightman said: "There has been a systemic failure over the awarding of English GCSE grades. It was well known that the introduction of this kind of specification required major adjustments to the awarding process and I am astonished that these changes were not put in place.
"Teachers and students acted in good faith and accept grade boundaries can change by a couple of marks, but to change by 10 or more makes a mockery of the system."
Lightman continued: "It is wholly unacceptable to leave the students and their teachers to pick up the pieces of a problem they did not cause. These changes implemented mid-year, without valid and reliable processes, must be reversed and arrangements put in place immediately to ensure that this does not happen again in future examination series.
"It is not acceptable or practicable to make the students resit examinations. Many will already have left their schools and decisions about apprenticeships and further courses of study are being made now as term starts and enrolment in most schools and colleges is already underway.
"If necessary ASCL will resort to a legal challenge to this unfairness. There are many lessons to learn but seeking to apportion blame at this stage will do nothing to resolve the urgent crisis affecting many thousands of students today."
A DfE spokesman said: "Ofqual is the independent regulator of exam standards. The Department is considering the issues raised by the report and we look forward to discussing these issues with Ofqual next week.”
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