When Emil Bednarski sat down in the exam hall in May and opened his GCSE maths paper, he had a shock of recognition. He had already seen question 21 the previous evening while searching online for practice papers.

Bednarski, 17, a pupil at Kensington Aldridge Academy (KAA), in west London, had been so puzzled by the trigonometry question he had shown it to a teacher on the morning of the exam and asked for help solving it.

In the months since, he has been accused by the exam board of cheating and “malpractice” and has been stripped of his GCSE maths results across three papers, leaving him unable to take A-level maths as he had hoped.

Bednarski and his family are calling for the exam to be re-marked for all candidates to exclude marks for that particular question, arguing he cannot be the only teenager who found it online.

It has since emerged that questions from the Edexcel maths paper, which tens of thousands of children sat that day, may have been leaked online before the exam started.

Bednarski is a high-achieving pupil described by the head of school as “hardworking” and “expected to do well”. He excelled at maths.

Keen to squeeze in some last-minute revision the night before his exam, he searched sites including Reddit, TikTok and 4chan for past papers, and found the question that appeared in his exam. The next morning, he approached a teacher to ask their advice, which he says clearly shows “I had no ulterior motive [and was not] trying to cheat. I was not trying to hide my awareness of this question.”

The teacher who helped him to solve it later realised the question was appearing in that day’s exam and told KAA’s head,who raised the alarm with the exam board, Pearson. After the exam, Bednarski assumed the board might have repeated a question used in previous years.

Within days, though, he was contacted by investigators from Pearson. Bednarski explained how he found the question, showing them messages and searches from his phone. He told them he and his friends had become aware that social media sites were selling papers online after the exams had finished, including on TikTok’s For You page, which suggests videos to the user. He also showed the investigators his bank account, to prove there had been no payments.

The investigators told the teenager: “It looks like in this case . . . a question has been made available before the exam was due to be sat . . . that’s the primary purpose of our investigation, we are focused on how this came to be shared, much more than we are that an individual may have received it.”

The school has supported Bednarski. In a statement, the head said: “I find itvery surprising that any student would volunteer to show us a question, if that question had been obtained illegally. I feel it is very difficult (and would be very unfair) to penalise him personally.”

Yet the teenager’s actions that day have resulted in him being stripped of his GCSE results across all three maths papers and adjudged to have committed malpractice — something he learnt only in August, when he went to pick up his results. He is now taking four A-levels at KAA but not further maths, which he cannot take as he has no GCSE grade.

His father, Cezary, says if that had led to the school refusing to allow his son to start in the sixth form, a maths GCSE often being a prerequisite, the situation could have had more serious consequences.

“Not guilty of anything, Emil would have been knocked off his A-level education pathway,” he said. “This would have had a significant impact on his prospects for the rest of his life, were it not for his school’s fairness and integrity.”

A letter from Pearson telling the teenager of the board’s decision says the finding of malpractice was made “on the balance of probability”.

The Bednarskis are fighting back, launching an appeal and lodging a complaint with the Joint Council for Qualifications. The teenager said in his statement to the exam board: “This whole saga, which in reality has nothing to do with me, placed me under massivestress. You started the questioning and investigations in the middle of my GCSEs.

You could have waited. I remember catching myself daydreaming midrevision sessions about what could happen if I am falsely accused.”

The family want Pearson to re-mark the paper for all pupils who sat the exam, excluding that one question and to apologise. “It would technically lower my mark because I got it correct, but I would rather lose three marks than my entire GCSE,” said Bednarski.

Cambridgeshire police confirmed last week that they are investigating a data breach involving Pearson and another exam board, OCR. In a statement, the force said the boards had exam papers “extracted from their systems and sold online” and that it was working with the National Crime Agency and the Department for Education. It added that a 16-year-old boy had been “arrested on suspicion of theft, fraud and computer misuse and released under investigation”. No charges have been brought.

The number of leaks of exam papers has grown since the advent of the internet and social media. Last summer, there were 28 leaks of material, including a Pearson GCSE maths question that appeared on social media before the exam. According to the annual report of Ofqual, the exam regulator, the board quickly identified the person involved and carried out “immediate inspections and extra supervision of exams”.

In previous years, exam boards have had to rewrite papers after they were stolen from vans or lost.

A spokeswoman for Pearson said: “We cannot comment on individual investigations — however we take all malpractice allegations very seriously and our expert team investigates each and every one. Malpractice is extremely rare. We have well-established processes in place to ensure fair and accurate results.

“All exam boards use analysis during and after marking to look for tell-tale signs of malpractice in exam papers — both at an individual and cohort level. Our review found no evidence that would require an adjustment to the marking or grading of exams.”