As few as 200 teenagers could gain a clean sweep of top grades in this year's GCSE exams, a study predicts.
New grades are due to be awarded for the first time this year in many GCSE courses - including key academic subjects such as science, history, geography and languages.
But few youngsters are likely to gain the highest result - a grade 9 - across all of their courses, according to research seen by the Press Association.
The paper published by Cambridge Assessment predicts that, among those taking at least eight GCSEs, between 200 and 900 will walk away with grade 9s in all subjects.
School leaders said the findings, which come just weeks before pupils learn their GCSE results, show achieving straight top grades under the new system is "extremely difficult" and will be a rare achievement.
The study used GCSE data from 2016 to predict the number of teenagers that are likely to achieve straight grade 9s in this summer's exams.
The most conservative estimate, based on the numbers that scored all A* grades, suggests that around 200 candidates taking at least eight GCSEs will score 9s across the board.
The other two predictions take a more speculative approach, and take into account the varying abilities of those who scored A* grades - such as those scoring at the top end of the grade.
In these scenarios, the study concludes that up to 900 students could walk away with straight grade 9s.
In 2016, around 2,000 students gained A* in at least eight GCSEs, the author said.
More than half a million teenagers in England alone sit GCSEs - which are also taken in Wales and Northern Ireland - each year.
Study author Tom Benton said: "Students should not be despondent, you do not need a clean sweep of grade 9s, and almost nobody is going to do it. You're talking about the top 0.1%.
"If you don't get a clean sweep it's not going to matter, there are still going to be plenty of options."
Mr Benton said one estimate, made last year, that just two pupils would get all grade 9s could have left pupils feeling discouraged, but the numbers are highly unlikely to be this low.
"Getting a grade 9 is a bit harder than an A*, but it is achievable," he said. "It's fine for students to have it as an ambition, but don't get disheartened if you don't get them across the board."
Geoff Barton, general secretary of the Association of School and College Leaders, said: "This research confirms that achieving straight Grade 9s is extremely difficult and will be a rare achievement.
"But we need to avoid becoming too obsessed with Grade 9s as a barometer of excellence.
"We are sure that this year's results will show many young people doing very well across a range of grades and all deserve congratulations."
Under major reforms, GCSEs have been toughened up, with less coursework and all exams taken at the end of the two-year course, rather than throughout.
New 9-1 grades have replaced the old-style A*-G system, with the first results - in English and maths courses - awarded last summer.
This summer sees the first time new grades will be awarded in most other subjects.
Fewer candidates are expected to achieve a 9 in each subjects than the proportion who gained an A*, due to the deliberate move to change the system to allow more differentiation, particularly between the brightest candidates.
Whereas before the highest grades were A and A* - this is now split across three - 7 to 9.