Nearly half of all private school pupils' A-level entries this year were awarded A grades, figures from the Independent Schools Council indicate.
ISC says 74.3% of private school entries were awarded As and Bs
The ISC says 47.9% of entries were awarded As and just 0.6% of entries failed to get at least a grade E.
The national average for A grades - including independent schools' entries - was 24.1%, while 3.4% failed to get an E, the lowest pass grade.
The ISC wants A-levels to be reformed to incorporate harder questions.
Ministers are already considering how to make the exams more challenging and, perhaps, having a new A* grade.
Officials are debating whether to introduce harder questions to A-level papers taken by all candidates in a subject, or to set the more challenging questions in a separate, optional section of the exam.
The figures from the ISC are drawn from the results of 31,700 pupils in 484 independent schools in England and Wales.
They show that three-quarters of grades in independent schools - 74.3% - were As and Bs this year.
The ISC chairman, Edward Gould, said A-level questions should be toughened up.
"The issue of a lack of differentiation amongst those gaining the top grade continues to cause concern particularly to universities," said Mr Gould.
"It is to be hoped that any proposals for further differentiation at the highest level will be linked to more challenging and demanding questions and not statistical slicing of the current assessment regime."
But the Department for Education and Skills said the A-level was a "tried, tested and trusted qualification".
"We recognise that as more young people make the grade we need to find ways of better differentiating between candidates," said a spokesman.
"That is why we are taking forward a range of measures to both stretch our brightest pupils and help universities and employers choose between the best.
"These include introducing tougher A-level questions, an extended project and providing universities with A Level unit grades."
Official statistics for how pupils have performed will not be published until October. The results released last week related to entries rather than candidates.
Last year, 27.1% of pupils in independent schools gained three or more grade As at A-level, compared with 8% of pupils in all state schools.