Ministers are facing renewed calls to update A-levels in light of the 23rd improvement in results in a row, with 22.8% of entries getting an A grade.
David Cameron said grade quotas should be investigated
Shadow education secretary David Cameron said a quota system, where only a prescribed number of students get each grade, could be looked at.
The National Union of Teachers and the Liberal Democrats called for A-levels to be replaced with a broader diploma.
But Education Minister Lord Adonis said the exams would remain in place.
The A-level pass rate overall improved this year by 0.2 of a percentage point to 96.2%.
Mr Cameron said it was difficult to say if exams were getting easier, adding: "It's different because there is more continuous assessment, there is more coursework and it is more modular than it was 20 years ago."
Exam results have improved every year since quotas for each grade were removed in 1982.
Mr Cameron told BBC Radio Four's Today programme that one option would be having a quota system, so a set number of students would achieve the top grade.
Earlier this year, ministers in England rejected plans to replace A-levels and GCSEs with a diploma system for 14 to 19-year-olds.
This would have included more vocational work and a greater breakdown of grades, showing results of individual course modules.
It also proposed extra, harder questions for bright students, allowing them to gain an A+ or an A++ grade.
In recent months the government has agreed to offer module grades to universities.
Lord Adonis also said the actual marks could also be made available if that would help admissions tutors to distinguish between the brightest students.
Steve Sinnott, general secretary of the NUT, said: "There is need for reform of our examination system but not as a result of any dumbing down but because of the changing demands of modern society.
"It is a pity that Lord Adonis appears to rule out the diploma option. That is very short-sighted."
Liberal Democrat education spokesman Edward Davey said: "Major reform of A-levels can't be ducked and Labour's refusal to act is the worst kind of political cowardice.
"Patching and mending a 50-year old exam system is not the answer.
"A diploma would stretch our brightest, broaden education and end the vocational-academic divide."
The general secretary of the Secondary Heads Association, John Dunford, said: "There remains a strong case for incorporating A-levels into a diploma and in the long term the name 'A-level' will disappear.
"Over the next year we will be pressing the government on this."
His counterpart at the National Association of Head Teachers, David Hart, said: "The government has to listen to the demands from an education world that is seeking a tougher and broader test of the country's able students.
"This would indeed enable the UK to compete effectively in increasingly demanding international markets."