As a result, five sixth-formers have had grades in information and communications technology raised after a complaint against the Assessment and Qualifications Alliance was upheld by an appeals panel. The Examinations and Appeals Board found that AQA had broken its own rules and those laid down by the Government when it undertook external moderation of work from Beauchamp college in Oadby.
The panel said the moderation process was flawed and said that repeated errors made by the exam board were "most regrettable".
Marks given by the board for some students' coursework were four grades lower than those awarded by the college's teachers.
But the decision comes too late for students hoping to use it to boost university applications.
It was third time lucky for Beauchamp, which had appealed on two previous occasions over problems with coursework. One earlier appeal led to marks for 101 candidates being increased, although only one student received a higher overall grade.
The appeals board praised measures taken by AQA to prevent similar mistakes in future.
Mike Vybiral, vice-principal at Beauchamp, said: "In all probability there are students at university now who would have benefited from these grades."
But he said the college wanted to put the episode behind it, and described measures taken by AQA as "positive for future generations".
An AQA spokeswoman said: "In cases like these we would always review our procedures to learn the lessons, and ensure that such a situation is not repeated."