Dozens of GCSE and A-level grades had to be changed after marking errors left candidates with the wrong results, an exam board admitted.
Around 250 students are thought to have been affected by the mistakes, with some pupils given results that were wrong by up to two grades.
The OCR exam board said it was "disappointed" to confirm that a number of grade changes had been made and said the errors were "unacceptable." Mistakes were made by examiners in transcribing and adding up marks in last summer's exams, OCR said.
As a result, four examiners have had their contracts terminated, and 78 others have been told to improve their performance. In total, 15 A-level grades have been improved, with 28 AS-levels, 37 GCSEs and 34 GCSE short courses.
In a further 81 A-level and 56 GCSE cases, changes have been made to units taken last year, and will be taken into account in final qualifications, the awarding body said. In most cases, the changes meant an improvement of one grade. But there were five students who saw their results go up by two grades.
Most of the GCSE changes were in Religious Studies, while the majority of A-level changes were in PE. It is not yet known if students missed out on university places after being awarded the wrong A-level grade. An OCR spokeswoman said that if students come forward, their results will be looked at on a case by case basis.
OCR chief executive Mark Dawe said: "Following the relocation of script handling to our new warehouse for 2011, I am disappointed to have to confirm a number of grade changes as a result of the discovery of clerical errors made last year.
"I would like to reassure students taking examinations for the first time this year (2012) that I have had our systems significantly improved and this issue cannot and will not affect them.
"Mistakes were made by examiners in the transcribing and totalling of marks. These are unacceptable and OCR apologises to students, schools and parents that our high standards for quality and integrity were not met by some individuals during routine processes.
"I have taken a number of corrective actions to address these matters including terminating four examiners' contracts, placing another 78 on notice to improve and instituting a new clerical checking regime. It is our job to ensure students get the right results that correctly reflect their knowledge and hard work."