A bright GCSE student was told by teachers she would have to dumb down to pass her exams.

Katie Merchant, 16, was marked down for giving a sophisticated answer in her mock Latin exam at Brighton College.

She achieved an A* - the highest mark possible - but lost marks on one question because her answer was too sophisticated.

Teachers warned her she would be similarly penalised in the real exam.

Katie expressed her disappointment in a letter to her Brighton College headteacher Richard Cairns.

Mr Cairns said examiners often marked papers in subjects they did not know a lot about, so he warned his pupils they might know more about the subject they were being tested in than the examiner.

He said: "The very brightest are definitely constrained by the marking schemes."

He said exam boards rewarded the highest marks for prescriptive answers containing key words, which means a pupil who displays originality is penalised.

"And the quality of examiners is not what it ought to be," he added.

Katie's experience backs this up.

In her letter she wrote: "I feel the entire exam structure is geared towards the standardisation of each pupil regardless of individual talent and ability and find this most discouraging."

Mr Cairns said the problem affected all exam boards.

He said markers rewarded children for thinking "mechanistically" rather than for thinking "outside of the box".

He went on: "Children are working harder than ever before. There's a momentum, a desire to do well. We're getting very good at teaching children to pass exams but less and less good at teaching them to think laterally."

Brighton College is addressing the problem, Mr Cairns said. After consultation with Oxford and Cambridge it is reducing the maximum number of GCSEs pupils can take from ten to nine, and making time in the curriculum for critical thinking.

Mr Cairns said: "There was a sense in the 1970s that teachers were doing what they liked. The good thing now is that through league tables, teachers are accountable to their pupils.

"As a result of this, teachers want more and more information about how to achieve an A* and this encourages exam boards to be more prescriptive and that has killed off independent thought."

The problem is compounded by poor quality examiners, he said, with examiners marking subjects they are not qualified in.

He said: "In my own subject, 16th Century history, there are people marking it who don't know the subject.

"I tell my students, 'You must expect the examiner to know less than you. He or she will be working to a rigid marking scheme and they need to look out for key things whether or not they're actually relevant'.

"It's something the Government and exam boards have got to address. They need to insist all examiners are qualified in that subject."