The new-style GCSE exams in England are the most difficult since the end of O-levels in the 1980s, according to an independent school leader.
The first results of revised GCSEs in English and maths will be published this week, with a grading system using numbers from 9 to 1.
Barnaby Lenon, chairman of the Independent Schools Council, says they will stretch the most able students.
But he urges universities to be cautious about using the top grade 9.
"Universities should not consider the distinction between an 8 and a 9 worth making until they have evidence that it does indicate something," Mr Lenon writes in an article to be published later this week.
"After all, 95% might get you a grade 8, 96-to-100% a grade 9. Does the grade 9 student have greater intellectual ability and academic potential or are they simply better at writing fast, or better at checking for silly errors?
"Only time, and analysis of results, will tell."
The new-style exams, beginning with English and maths, will no longer use coursework or modules, but will be graded on final exams.
Mr Lenon predicted that schools that had relied on coursework to boost results could "suffer a fall in grades this year".
He argues that coursework was an "unreliable measure" of ability, "much of it had little value" and it could too easily be "influenced" by teachers or parents.
There will also be changes to the syllabuses to make them more demanding.
While much of the attention will be on the new grading system, Mr Lenon says it is important not to miss the scale of change for the qualification.
"They contain questions of a level of difficulty that we have not seen since the abolition of O-levels in 1987," says Mr Lenon.
This is intended to stretch pupils in England so that they can catch up with the standards of pupils in east Asia, he says.
But if the new exams provide more challenge for the most gifted pupils, he says the impact at the average and lower ability end remains uncertain.
"Raising the bar" will not necessarily help these pupils, he says, unless they have the support to "jump higher".
"It is the quality of teaching of less able or less diligent pupils that will help them to succeed in their GCSEs."
Head teachers' leader Geoff Barton said that schools would be concerned about "volatility" in the results of individual schools, below the surface of national results.
New exams 'demanding'
Mr Barton, head of the ASCL head-teachers' union, said there should be caution about interpreting and comparing the results of such a different form of GCSE.
Teachers and pupils would have to adjust to a different style of qualification, he said, and he warned against people "springing to judgement" over unanticipated results.
John Blake, head of education at the Policy Exchange think tank, backed the changes.
"These new GCSEs are demanding, and rightly so," said Mr Blake.
"The comparison with O-level is a good one - those qualifications were designed for our most academic children, and as we improve our curriculum and our expectations of all our young people, it is right they be the benchmark for success."
GCSE results are to be published on Thursday - including the first wave of new 9-to-1 grades.
There have been warnings of confusion over numerical grading and questions about a system that will have two different types of pass grade - with grade 4 a "standard" pass and grade 5 a "strong" pass.
The Institute of Directors warned last week that some employers might not understand the new grades and would see them as "gibberish".