More than one in 10 pupils in England achieved five A* or A grades in 2000. Eight per cent of pupils in comprehensives reached this level, compared to 48 per cent of grammar school students.
However, GCSE league tables to be published in two weeks' time will only show the proportion gaining five A* to C grades.
Professor David Jesson, of York University, whose research has been used by ministers to demonstrate the success of specialist schools, claims it is impossible to differentiate between top schools under the current system. "In some secondaries, particularly grammar schools, most candidates achieve five A* to A grades."
He cited Kent, which has about 70 comprehensive and 30 grammar schools:
"The grammar schools look great compared to the others. If there was an additional table based on five A* and A it would give parents a better picture."
School GCSE point scores are contained in the performance tables but Professor Jesson said grade data was easier to understand.
Tim Andrew, head of Chesham grammar school in Buckinghamshire, where 98 per cent of pupils reach the five A* to C benchmark, said: "Schools already look at A grades internally."
Fred Naylor, secretary of the National Grammar School Association, also wants an A* to A or A* to B performance indicator.
Department for Education and Skills advisers and officials from Ofsted and the standards and effectiveness unit are discussing the idea.
The A-grade measure has been put forward after proposals to create a distinction A-level, which would have challenged the brightest sixth-formers with harder questions, were shelved by ministers. Heads, the exam watchdog and exam boards opposed the plan.
Government plans for 14 to 19 reforms should be published later this month. Ministers have also gone back to the drawing board on the matriculation diploma. While they remain in favour of an overarching qualification, the design in the Green Paper will be rewritten.