Video report by ITV News Correspondent Ben Chapman
Overall UK GCSE pass rates have fallen this year after the biggest shake-up of exams in a generation.
Among 16-year-olds in England, around 18,600 maths entries scored a 9 - the new highest grade, while almost 31,000 achieved the top mark in the two English GCSEs combined.
English and maths - key GCSEs for all teenagers - were scored this way this year.
But the news grades system will be rolled out to all other subjects by 2020.
Today's figures show that across England, Wales and Northern Ireland, the proportion of entries scoring at least an A grade - or a 7 under the new system - has fallen by 0.5 percentage points to 20% compared to last summer.
While the percentage gaining a C or above - or a 4 under the new system - is down 0.6 percentage points to 66.3%.
Girls outperformed boys in 9 grades in both English GCSEs, while boys did better in maths at the highest result.
The grading switch is part of wider reforms designed to make GCSEs more rigorous and challenging.
A grade 7 is broadly equivalent to an A under the old system, while a 4 is broadly equivalent to a C.
It is predicted that many teenagers who would have gained the highest possible grade last year will not do so this summer.
Schools Minister Nick Gibb said: "A new grading system was needed to distinguish between the old and the new reformed GCSEs.
"The new grading system also provides stretch for the highest performers by showing greater distinction between the top marks."
But school leaders warned the reforms are already causing teenagers more stress and anxiety, and this is likely to increase as more subjects switch to the new system.
Association of School and College Leaders general secretary Geoff Barton said: "We know from numerous reports that there is a rising tide of mental health issues among young people and we are concerned the new exams will make the situation worse.
"The new GCSEs are more challenging, and there are more papers, and this is putting severe pressure on young people.
"We support a robust qualification system, but it has to be balanced against the welfare of young people, and we are not sure the balance in the new system is correct."