Tony Deacon, 48, had approached history teachers at the school last GCSE results day, and was lent a textbook by head of history, Hermione Baines.
He sat his final exam on the same day of a job interview for a new role at a nearby primary, receiving a call just a few hours later to say he had been successful.
He said: “I never thought I’d be able to do it. I’ve proved myself wrong -– it just shows you’re never too old to learn.”
Mr Deacon, who left the school in June after eight years, said he had never been interested in the subject at school.
As an adult, he added, he had wanted to learn and had made sure he put in the hours. In a year’s time, he said, he may try for an A-level.
“It just felt right,” he added. “I thoroughly enjoyed it. Don’t ever let anybody say you can’t do something. Everybody can achieve what they want to do.”