A pupil was stripped of his maths GCSE after a question he found online while revising appeared on his exam paper.
Emil Bednarski, 17, who studies at Kensington Aldridge Academy, had asked a teacher for help with the question just before sitting the exam.
The school reported it to exam board Pearson, who refused to issue his GCSE. On appeal, he was awarded his GCSE but a Grade 5, as one paper was discounted.
Emil said he had "no need to cheat at maths" and plans to resit next month.
His father, Cezary Bednarski, said they are now seeking an apology from Pearson - the UK's largest awarding body - and compensation.
Had Pearson allowed all Emil's maths papers to be marked, he would have achieved a Grade 8 or 9, Cezary said.
The school has declined to comment, but Mr Bednarski said he understands why they had to report the incident, adding they have been supportive of his son.
In a letter stating the result of the appeal, Luke Jenkins of Pearson said it was clear Emil had gained access to the question before the exam, but it was not clear if he had known it was going to be on the paper before he took it.
He said after realising he had seen an exam question, instead of alerting the exam board, he deleted the image containing the trigonometry question.
Mr Bednarski said Emil was "knocked off his A-level education pathway", adding that this was "likely to have a significant impact on his prospects for the rest of his life."
Emil said before any exam, he spends lots of time preparing and searching for past exam questions and trying to answer them.
"This I did preparing for my GCSE exam in maths too," he said. "I am very serious about my education, and my educational attainments."
In a statement to Pearson, he said: "This whole saga, which in reality has nothing to do with me, placed me under massive stress.
"You started the questioning and investigations in the middle of my GCSEs. You could have waited. I remember catching myself daydreaming mid-revision sessions about what could happen if I am falsely accused."
A spokeswoman for Pearson said it took all malpractice allegations "very seriously" but had found no evidence of a wider leak of questions online.
"All exam boards use analysis during and after marking to look for telltale signs of malpractice in exam papers - both at an individual and cohort level," she said.
"Our review found no evidence that would require an adjustment to the marking or grading of exams."
Kensington Aldridge Academy in west London is a non-selective, co-educational state school, which is recognised as one of the top academies in the UK.
In June 2017 it was affected by the Grenfell fire, which led to the deaths of 72 people, including four pupils.
Fifty pupils were made homeless and the whole school had to relocate following the tragedy.
The following year it was awarded TES Secondary School of the Year.
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