More than 50 sixth formers have been told the A-levels they sat in January this year were wrongly marked.
Exams have been wrongly marked for second time at St George's
The discovery by St George's School in Harpenden, Herts, came just months after 100 of its pupils were given the wrong GCSE grades in summer exams.
Head teacher Norman Hoare, who appealed against the marking, said he did have some sympathy for the exam board.
"It is inevitable mistakes will be made given the scale examinations when thousands are marked," he said.
He paid tribute to examination board AQA for remarking the scripts promptly as soon as they realised something was wrong.
He said: "They remarked the whole batch and I believe they would have done the same thing for any school."