Pressure on exam boards to reduce the number of top scores at GCSE "left the dreams of youngsters in tatters", according to a teacher.
In an open letter to education secretary Michael Gove, teacher Chris Edwards described what it was like to see students lose places in colleges because of the unexpected fall in their grades.
"On opening the envelopes and seeing their D grades, each and every one of them covered their faces due to the shame that they felt," he wrote.
"I spent the vast majority of the morning consoling students, who worked more than hard enough to achieve a C grade in English, had been predicted a C grade in English and effectively had earned a C grade in English, but had been credited with a D grade, thus scuppering their chances of going to a college which had conditionally accepted them based on their predicted grades."
He added: "They can't understand why someone would want to play around with their futures in such a cruel way and we, as teachers, should not have to be the ones to explain it to them.
"You have not simply moved the goalposts. You have demolished them, sold off the playing fields where they once stood and left the dreams of these youngsters in tatters."
Critics suggested the decline in A* to C results yesterday – the first in GCSE history – was the result of new rules introduced to combat grade inflation, which Gove believes is discrediting the exam system.
In all subjects, the number of students achieving the A* grade was down 0.5%, while the number earning marks between A*-C was down 0.4%.
"It appears that today you got what you wanted," Edwards wrote.
"The statistics show that GCSE passes are down and to you, statistics is all they will ever be. But to me and every other teacher I have had the pleasure of working with, these children are not statistics.
"They are young people who you have betrayed and will forever be affected by the contents in that envelope which they opened today."
Edwards, who posted his open letter online, could not be reached for comment.