An "excellent" GCSE student had a mark taken away for giving an answer which a teacher said was "too sophisticated".
Katie Merchant, 16, of Brighton College, missed a "key word" in a Latin mock exam, although she showed more than enough knowledge for a full mark.
Headmaster Richard Cairns said the OCR exam board's assessment scheme, which the school used, was "too mechanistic".
An OCR spokeswoman denied bright pupils were held back and said the system was intended to be fair to all.
Mr Cairns said marking schemes - which tells an examiner how many marks to allocate for the various elements of an answer - "frustrated" bright pupils.
He is reducing the maximum number of GCSEs a pupil at the independent school can sit from 11 to nine, to free up more time for "critical thinking".
Mr Cairns said: "The teacher had to take a mark away from Katie even though her answer was excellent.
"We have to follow the exam board's guidelines but it's all so mechanistic."
Katie was still awarded a top A* grade, but Mr Cairns said: "There's no room for individuality. It's like encouraging children to dig up roads when they could be designing bridges."
An OCR spokeswoman said that, as the board had not looked at the school's marking of Katie's paper, she could not comment on the particular case.
She added: "OCR's mark schemes are there to ensure that all candidates receive a fair mark for their work.
"Candidates will always receive credit for answering a question well and our mark schemes do not hold back brighter students."