Details of A-level grade boundaries for all papers set by two exam boards have been leaked on social media a day before the results are published.
The documents reveal that A-level maths candidates needed little more than half marks to get a grade A in papers set by both Pearson/Edexcel and OCR.
Exam boards send marking schemes to schools in confidence ahead of the results to help them prepare students.
They said confidentiality was usually respected.
A-level students are due to get their results on Thursday.
New maths exam
The leaked Edexcel documents, first reported in the Daily Telegraph, show A-level maths candidates needed 55% to get the A grade, while candidates with the OCR board needed 54%.
Edexcel's biology students needed 56% (167 marks out of 300) for an A, while OCR's candidates needed 59% (158 out of 270).
In physics, candidates needed 59% (176 out of 300) for a grade A with Edexcel, while with OCR, this was 76% (204 out of 270).
In English literature, candidates with the OCR board needed 89% for an A grade - significantly higher than those with Edexcel, who needed 69% (208 marks out of 300) to get A.
This summer is the first time that grades for new-specification A-level maths are being awarded to the vast majority of students.
The exam was reported by students to be very challenging, but the exam board had said before the leak emerged that it was clear the paper was of an appropriate standard.
Exam boards set grade boundaries once all the results are in.
They take into account the predicted achievement levels of the cohort taking the exam and the difficulty of the paper, in an attempt to keep standards the same from year to year.
Responding to the leak, a spokesman for Pearson said: "Our systems are working as they should and the information was shared today via a password-protected, secure website.
"Boards do ask schools not to share this widely to avoid unnecessary stress for students awaiting their results.
"Schools are trusted to treat the info confidentially on behalf of their students - and the vast majority do."
A spokesman for Cambridge Assessment, which owns the OCR board, said: "We provide schools and colleges with results information, including grade boundaries, the day before results day on a special site which is accessible to exams officers.
"Grade boundaries are then released by schools to their students on results day and we publish them on our website.
"We do it this way to minimise the chance of students feeling anxious if they see grade boundaries without their results — which can lead some to jump to the wrong conclusion."
General secretary of the Association of School and College Leaders Geoff Barton said: "We are extremely disappointed if grade boundaries have been leaked ahead of results day."
But he said was a pointless exercise because the purpose of grade boundaries was to account for differences in the difficulty of papers so that students were not disadvantaged from one year to the next.
"We would urge students against losing sleep over grade boundaries and to wait for their results tomorrow."