It is "morally wrong" to undermine teenagers' GCSE efforts with talk that the exams are too easy, a school leader said on Monday.
Pupils are working extremely hard, and should not be told that their achievements are not enough, according to Brian Lightman, general secretary of the Association of School and College Leaders (ASCL).
With less than a week to go before youngsters receive their GCSE results, now is the time to celebrate their success, he said.
"I think we've got to be very careful with the message we send out about quality, it's potentially very dangerous to undermine confidence in the system.
"The fact of the matter is children are working extremely hard and teachers are working extremely hard to get them through exams."
Children are doing "everything asked of them," Lightman said.
"They are working really hard and taking exams that are rigorous. They shouldn't be told that their achievements are not good enough.
"I think they get terribly frustrated by this. It's just wrong, it's morally wrong to undermine their efforts."
Lightman said there were concerns about "comments that we need to have more rigour in the system and that exams are too easy".
More young people are completing their exams at 16, with higher grades than we have ever had before, he said.
"What we should be doing next week is celebrating achievements and the enormous effort that has gone in by pupils and teachers," he added.
The future of GCSEs has been under the microscope in recent months.
It was reported in June that Education Secretary Michael Gove was considering proposals to ditch GCSEs in favour of a return to O-level-style qualifications, with less able pupils taking simpler CSE-type exams.
The leaked plans resulted in an outcry that it would lead to a two-tier system and thousands of teenagers being branded as failures.
Gove later said that he would like to see all students sit O-level-type exams at some point in their school career.
He has argued that a two-tier system already exists and radical changes are needed to make exams tougher and ensure that the UK keeps up with other nations.
In a speech in June he said: "I want us to ensure that in the next 10 years at least 80% of our young people are on course to securing good passes in properly tested exams in maths, English and science - more rigorous than those our children sit now."
One expert today predicted that increases in GCSE pass rates will slow this year.
Professor Alan Smithers, of the Centre for Education and Employment Research at Buckingham University said that passes at the highest levels have "bounded upwards" ever since GCSEs were introduced.
This has been fuelled by increases in girls' performance, he suggested.
Prof Smithers also said that schools put a lot of effort into making sure that pupils score at least five C grades, including English and maths, as they are judged on these results.
"There's a lot of effort going into improvement there," he said.
At the same time, similar to A-levels, Ofqual has told exam boards that they will be asked to justify any results that are widely different to previous years, in a bid to tackle grade inflation and ensure that results are comparable.
Prof Smithers also said: "Ofqual has expressed concern about science GCSEs, and it explicitly said it wanted more difficult science GCSEs.
"It's the first time it (new science GCSE) is being examined this year so we might expect to see a fall in various grades in science.
"If you put all that together, I think the results this year will be similar to last year, with some improvement."
He predicted that that the A*-C pass rate would be "close to where it was, I would have said probably close to 70%".
"And I would have said A* to A, 23.5%," he added.
Last year 69.8% of GCSE entries gained at least a C grade, and 23.2% got an A or A*.
"Ofqual policy is if the boards can explain why the scores should go up, then that's perfectly ok, it's not a sort of straitjacket.
"Ofqual are keen to reduce the amount of inflation in the scores."
Girls may be outperforming boys at GCSE because they are better behaved, a poll of 538 children suggests.
The Plan UK survey found that half (51%) think girls get higher scores because they behave better in lessons and 49% said it is because girls concentrate more in school.
Last year, 66% of boys' GCSE entries scored at least a C compared to 73.5% of girls.