Dorrinda Graham, who took her A-levels at King James school this summer, says she has lost out on a place at university because of problems with the psychology exam.
She gained a grade D for her coursework, despite being predicted a B. This reduced her overall mark from a B to a C. She was awarded Cs in English language and English literature.
She said she would have been confident of being accepted at Leeds university to read psychology if her mark had been one grade higher.
She said: "I just feel knocked back because I did so much work. I made sure I gave the information which is needed for a good grade, then when it comes round to it they change the rules. They had a grade boundary system which we all worked to. When it came out, it was changed. So what would have got a B now gets a C. It's just ridiculous."
This appears to be the second year running Dorrinda has suffered at the hands of the exam board. Last year, she had to appeal to the board after getting an E for the coursework element of psychology AS-level. In February, after a re-mark, this improved to a B. "I have no faith in the marking system any more," she said.