John Gretton is running late, says his PA, with a smile that suggests she has to say that a lot. Before long, though, the 63-year-old headteacher bursts into his office, already talking at full speed. He was caught up with a set of parents who called in unexpectedly, he explains. They hadn’t been in the country for long, let alone the school, so he didn’t want to cut the meeting short.

His attention then turns to the past, as he reads a card from a former pupil now in her early 30s. She has written to say that she has lovely memories of the school and of her favourite teacher, Mrs Marsden - who just happens to be the headteacher’s older sister. On cue, Pat Marsden, now assistant head, arrives in her brother’s office. The thank-you card is passed around and they chat fondly about their former student, now a mother, who still lives nearby.

The brother and sister team are retiring this term after 40 years together at St Anthony’s Roman Catholic Primary School in Wythenshawe, near Manchester airport. Cards like this one are flooding in, taking over Mr Gretton’s whiteboard. A celebratory meal is planned to mark the occasion, as well as a school event towards the end of term.

Over a plate of corned-beef sandwiches, Mr Gretton confesses that he is already worried about his emotions getting the better of him. “I’m a real cry-baby,” he laughs. “I’ll be holding back the tears.”

“I am more likely to hold it in,” says Mrs Marsden, as her brother nods. “But, then, I’ll probably cry when I get back home.”

It is bound to be an emotional time for the siblings. Not only have they been at the school for almost their entire working lives, but they also grew up nearby - in Wythenshawe’s Newall Green area, with their three brothers and a sister.

The head and assistant head have since become firmly rooted in the community. Both live within minutes of the school and attend the parish church. During their time at its helm, St Anthony’s has become one of the top-performing schools in Manchester. It remains oversubscribed, despite catering for 670 pupils.

The school was credited by Ofsted for fostering a strong family atmosphere. Relationships - among teachers as well as between staff and pupils - are at the core of the school’s ethos. The pair have seen generations of families pass through the school and regularly meet former pupils as parents and even grandparents.

It is hard to believe that a brother and sister - with just one year between them - would willingly spend so much time together. But, even after all this time, they laugh at each other’s jokes, of which there are plenty, and one - usually Mr Gretton - finishes the other’s sentences.

Perhaps it has something to do with teaching being in their genes - their uncle was a teacher, and their grandmother and great-grandmother were headteachers in Ireland. “I remember going to our grandma’s rambling house in Manchester,” says Mr Gretton, sitting back in his chair and sipping a cup of tea. “We used to sit on the window sill with our two cousins and two Siamese cats. In the room would be all these relatives, drinking and smoking, telling stories and giving out opinions. And I think the interest in teaching came from that - the storytelling and the education.”

These days, it is often assumed that family connections in the workplace amount to nepotism. Deputy prime minister Nick Clegg recently called for an end to relatives securing work placements and improving the chances of the privileged few.

But Mr Gretton came to the profession by chance, three years after his sister. He had originally pursued a legal career, but decided to try his hand at teaching. By complete fluke, his last PGCE placement in 1970 was at his sister’s school, St Anthony’s. “As soon as I arrived, I loved it,” he says. “Sometimes you get a sense that something was just meant to be.”

After six years, the job of headteacher at the school became free. Having just finished his MA, Mr Gretton applied. “There were about 68 applicants - whereas now, there would only be 10,” he says. “But I had a wonderful interview - you just do sometimes. I became a head at 30.”

At the time, his sister was off on maternity leave and the transition to becoming the leader, particularly at a young age, was tough for Mr Gretton.

“In those days, people weren’t used to having such a young head,” he says. “They’re used to it now, what with all these fast-track schemes. But people are very fair and, on the whole, they judge you for what you do and what you are like. You just have to battle through and show them that you’re capable.”

When Mrs Marsden came back to work after having two children, her little brother was suddenly her boss. But she says that going from helping him with his PGCE to being one of his staff was never a problem. “I’ve always admired the way he’s tried to do the job,” she says. “He’s got a good sense of humour, but he’s always in control.”

As assistant head, she continues to teach Year 6. She also mentors new teachers and “makes sure everybody’s happy”. “John and the deputy head are very much in charge of the school, whereas I tend to ensure all the routines go well,” she says.

Mr Gretton may still be in charge, but the role of headteacher has changed over the years. When he started out, heads were almost entirely responsible for schools. But then boards of governors began to have more say, followed by the rising influence of teaching unions and teacher governors.

“After that, it was about parent power through parent-teacher associations and now the school council,” he says. “It is much more collegiate now in the senior management team, and I prefer it like that,” he says. “What I’m paid for is to be in charge of the leadership. I rely on all these other talented people to run the school. My job is to spot the talent and make sure it’s promoted and encouraged.”

The teachers at St Anthony’s do seem to be well taken care of. They talk fondly about their residential weekends away together, one of their favourites being a trip to Dublin a few years ago. These are opportunities for everyone from the administrative staff to the teaching assistants to get to know each other.

“It’s not much more expensive than doing separate Inset days throughout the year,” says Mr Gretton. He has also organised yoga classes for staff, he says, pointing to a yoga mat in the corner. “Every school should offer something to teachers,” he says.

After June, none of this will be the responsibility of the two siblings. Instead, they will focus their attention on planning a trip to South America, where they intend to visit a priest in Sao Paulo for whose charity they have raised money in the past, and to spend time in Argentina.

“I’d also like to start tracing our family tree,” says Mr Gretton of his retirement plans. “We’ve got 58 first cousins. Maybe I’ll go and see some of them.”

At 64, Mrs Marsden is retiring later than is usual. She had planned to leave school at 60 but then her husband passed away. “They were going to spend retirement together,” says her brother. “So I coaxed her to stay on by saying `just do another year’, and then `just do one more year’.”

When Mr Gretton called it a day, his sister decided to join him. “Teaching is physically and mentally very hard,” she says. “But also, I wouldn’t really like to work for a new head. It would be better for them to have a fresh start without me interfering.”

Mr Gretton agrees. Despite having shaped the direction of St Anthony’s over the past 34 years, he is happy to hand over the reins. “It will be good for the school, because there will be areas that we’ve not developed,” he says.

The school received an Artsmark Gold for encouraging pupils’ creative side by focusing on drama, music and theatre visits. “But somebody else might want to take it in a different direction, with a different emphasis,” says Mr Gretton. “Somebody younger will bring dynamism and will probably have more energy.”

Before lunch is over, conversation returns to the thank-you card. Mr Gretton remembers meeting the former pupil who sent it a few years ago. “She was working in a cafe and she seemed to be very happy with her children and living near her twin sister,” he says. “That’s all we want for our pupils - the most important thing in life is to find something that you love.”

His sister nods in agreement and looks up from reading the card. “We want our children to be happy and confident at school,” she adds. “The rest will follow.”