Police raided a private tutorial college in south London on Monday night and found about 10 pupils reading through the maths paper due to be taken by more than 300,000 teenagers the next day.
Police arrested a woman teacher and her husband who jointly run the college. The police acted after Edexcel received "reliable information" that a theft had occurred from Archbishop Lafranc school, in Croydon, Surrey. The woman also works at the south London school and had responsibility for organising community language exams and exams for external candidates.
David Clark, head at Archbishop Lafranc, said that the teacher was given limited access to a small number of exam papers which were at all times locked in a secure place.
But Edexcel's chief executive John Kerr accused the head of ignoring the rules. Exam papers are sent to schools in sealed bags and must be locked in a safe until the day of the exam.
Edexcel will now put in place special monitoring procedures to guard against future security breaches.
The exam board said there was no evidence that the students attending the tutorial centre, at Motspur Park, Merton, knew that they were being given live exam papers.
Their scripts will be marked by a senior examiner and compared with their past performance. A spokeswoman said the pupils "will not be penalised". The two adults have been released on police bail until July.
An allegation was made last summer that an Edexcel A-level maths paper was being sold on the black market. One student was disqualified from the exam but sufficient evidence to prosecute was not found.