This summer, new grades are to be awarded for the first time in many courses, including key academic subjects such as science, history, geography and languages.
It will see a traditional A*-G system replaced by numbers 9-1 to allow more differentiation, particularly between the brightest candidates.
But, research published by Cambridge Assessment has predicted, very few youngsters are likely to gain the top result across all subjects.
The study, using GCSE data from 2016 to predict this summer’s grades, has found that between 200 and 900 students will walk away with 9s in all subjects.
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.”
The findings come just weeks before pupils learn their results, and follows the first results in the new system - for English and maths courses - which were awarded last summer. Whereas before the highest grades previously were A and A*, this is now split across three - 7 to 9.
The most conservative estimate in the study, 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.
Two further 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, author Tom Benton said. But, he added, youngsters shouldn’t be dishearted by the findings.
“Students should not be despondent, you do not need a clean sweep of grade 9s, and almost nobody is going to do it,” he said. “You’re talking about the top 0.1 per cent.
“Getting a grade 9 is a bit harder than an A*, but it is achievable,” he added. “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.”
An Ofqual spokesman said: “Not all GCSEs are graded 9 to 1 this summer, so students may also get some A* to G grades too.
“We have not done any modelling regarding the numbers likely to receive all grade 9s in those subjects which are graded numerically this year and make no prediction of figures like these in advance of a future awarding process.
“Neither did we previously make these kinds of calculations for the number of A* awards at GCSE.”