The new GCSE grading system will put more pressure on young people to gain top marks, according to a leading headmistress.
Caroline Jordan, president of the Girls' Schools Association (GSA), said she was concerned that the change - particularly the new top grade for the highest performers - will have a negative impact on youngsters "with a tendency towards perfectionism".
Under major exam reforms, A*-G grades are being replaced with numerical 1-9 grading - with 9 the highest result.
The proportion of teenagers gaining at least a grade 7 is expected to be broadly similar to the proportion that would be expected to get an A* or A under the current system.
Around 20% of all results at 7 or above will be given a grade 9, exams regulator Ofqual has said - meaning it will be awarded to the very top performers.
In her speech to the GSA's annual conference, Ms Jordan, who is also headmistress of Headington School, Oxford, said: "The pressures on pupils today are enormous and the statistics on mental health must not be swept under the carpet.
"It worries me that the movement away from alphabetical grades at GCSE to numerical grades, and particularly the controversial grade 9, will place even more pressure on young people.
"Indeed, it seems certain that it will do just that, when you consider that the new grade 9 will be awarded to only 20% of those who would have achieved A* to A under the existing system.
"It's right that we have rigour. It's not right that we make our children ill in the process.
"I am worried for all those pupils with a tendency towards perfectionism, many of whom we know to be girls.
"Many of us are spending significant time introducing our highly aspirational parents to the reality that only the brightest of the bright will achieve grade 9 and helping them to understand that the days of all bright pupils getting 10 A*s are over. Ten grade 9s really will be exceptional."
She added: "Thankfully, girls' schools have the luxury of being able to design their entire pastoral and academic support around the needs of girls, so I know that we are well placed to manage the impact of grade 9 and encourage our pupils away from 'achievement at any cost'.
"Let's make sure we carry on helping them to maintain a sensible balance between study, extra-curricular pursuits, family time and time for themselves.
"It's good to aim high but students must also be realistic about results. That's why initiatives such as Little Miss Perfect at Oxford High School are so important, giving girls the strong message that it's okay not to be perfect all the time.
"Failure can be as valuable a learning experience as success."
Revamped GCSEs in English language, English literature and maths were introduced into England's schools in September last year and the first exams, with the new grading system, will take place next summer.
New GCSEs in other subjects are currently being brought in over a number of years.
A Department for Education spokesman said: "Our reforms have actually reduced exam pressure by scrapping January modules and removing resits from league tables so pupils have at least two full years of study before sitting an exam, rather than a constant treadmill of revision and testing.
"Our GCSE reforms will make them gold-standard qualifications matching the best education systems in the world and allow young people to compete in an increasingly global workplace."
An Ofqual spokesman said: "We understand that students will always want to strive to do the best they can, whatever grade they are targeting.
"We adopted the new GCSE 9 to 1 grading system following public consultation in part to help employers and those in higher education better differentiate between pupil performance.
"The change will mean about 5% of pupils will get a grade 9 on average across all GCSE subjects in future, compared with about 8% who get A* grades today."