Joe Mullens used to be responsible for the welfare of hundreds of murderers and rapists. Today, in a south London primary school, he’s dealing with snotty noses. Until four years ago Mr Mullens, 58, was one of Britain’s most senior prison governors; today he works just down the road from Wandsworth prison, where he started his career almost 30 years ago. These days he’s swapped the grim Victorian blockhouse of the jail for the mock-Gothic curlicues of St Anne’s Church of England primary school, where he works as a teaching assistant.

He surrendered a pound;90,000 salary for pound;6 an hour, but the switch was founded on his two guiding principles: a belief in the value of education, and in service to society. “I have seen what a difference education can make to someone in jail. I know how important it is to get it right,” he says.

After being made redundant by the private prisons company he’d been working for since 1997, Mr Mullens felt burned out and ready for a change. “I wanted a reason to get out of bed in the morning, and a job where I could give something back. I loved education, but teaching would have been too high pressure,” he says.

These days he still rises early, walking the two and half miles to work to keep in shape - an old habit dating from his days in the governor’s office.

On arrival, instead of reviewing overnight disciplinary breaches, he sits down with Year 5s who are struggling in maths.

“In the old days, prisoners would come to me with their worries and ask for my help. I would do what I could, or follow it up with a prison officer.

The best reward would be, ‘Thanks for sorting it, Guv’. In my maths group it’s the same thing.”

He describes his delight after one boy, who has behavioural problems and finds it hard to settle to work, finally grasped fractions. “His whole face lit up; it’s what makes this job worthwhile.”

Mr Mullens’s face brightens in similar fashion when he describes the work of the education department at Channings Wood, a medium security prison he ran in Devon from 1989 to 1993. “It was wonderful. We had nearly 500 men, including 60 to 80 foreign prisoners who spoke no English. They were so thirsty for education that they lapped it up.”

One of them, a young drug runner on an eight-year sentence, sent Mr Mullens a postcard after he was deported back to Colombia, thanking him for all he’d done and saying that he was now studying to become a doctor.

He sees strong parallels between school and prison; they are both “closed worlds” where an assumption that everyone knows the rules can lead to a breakdown in communication. He used to make a point of walking the landings of his prisons to reassure prisoners that he was interested in their welfare. Similarly, at St Anne’s he keeps an eye out for “problem kids”.

In the dining hall he zeroes in on Ben, a small child sniffling sadly amid a noisy group of boys, who “tends to get in a state over things - needs space”. Mr Mullens talks to Ben quietly for a moment, then settles him at another table. “He’s sensitive, and sometimes he just needs peace and quiet away from people. He’ll perk up now.”

Even while dealing with the most hardened lifers, Joe Mullens believed passionately in the importance of families. “Jail is catastrophic for them,” he says. “For a child, getting a letter from Dad on prison notepaper, with the censor’s marks on it, brings home the reality of what has happened, but it also keeps the father real for the child and the family real for the father. It will help him return to society.” For the same reason, he encouraged governors of prisons in the East Midlands, where he was area manager, to adopt a scheme where prisoners taped stories for their children to listen to back home. “You could see the good this was doing - to the prisoners, who felt valued, and to their children, who needed that contact.”

Having, in a sense, been a headteacher in his own prison, Mr Mullens admits it is hard not to proffer advice to his teacher, or indeed his head. “I do have strong views, but I’ve learnt to keep them to myself. Not being a manager is something I was apprehensive about, but I’ve got used to it,” he says.

Yvonne Norman, St Anne’s head, admits she was intrigued by his application.

“I thought at first that it was a bit weird. But he explained his motivation at interview,” she says.

Not much phases Joe Mullens. Back in the dinner hall he eyes a lanky Year 6 boy swinging on door handles. Joe’s expression toughens and suddenly one can see the governor in him. “Benny, out you go.” He doesn’t raise his voice, but the effect is immediate: “Yes, Mr Mullens.”

For the final session of the day he helps in the nursery, chatting to the children and dishing out milk and fruit. He’s also on the lookout for runny noses. “I can’t bear to see kids with streaming noses,” he says, sounding just like the grandparent that he is. “Come here, baddy, it’s off to prison with you,” says Tim, a stocky four-year-old who marches up in a policeman’s helmet, brandishing toy handcuffs. “Oh dear,” says Joe Mullens. “Not back there again.”

All the children’s names have been changed