Mr Duffield by day, Dad at home time
But Ian Duffield insists there is nothing unhealthy in his family relationships.
"Sometimes the kids forget themselves and call me 'granddad'," he said.
"But they're sensible. Most of the time they call me 'sir', just like any of their friends."
Mr Duffield is a PE teacher at Whitby community college, in North Yorkshire. He is the patriarch of a family represented in the school over three generations.
Among the Whitby pupils taught by the 62-year-old are his grandchildren Laura and Alec. And, working with him in the PE department, is his 36-year-old daughter Julie Mastrolonardo, who is Laura and Alec's aunt.
Mr Duffield has taught at Whitby for 10 years and encouraged his daughter to accept the position she was offered there 18 months ago. But he insists that, on the whole, he resists the temptation to dispense fatherly advice.
"She's her own person," he said. "So I don't offer a great deal of teaching advice. She performs very well under her own steam."
But Ms Mastrolonardo found it more difficult to adjust to sharing lesson time with her father. "At first, working together was like having an inspector in. But you get used to it."
She now works hard to separate her professional relationship with her father from their personal one. She does not turn to him for lesson tips.
Nor does she ask him to fill her in with staffroom gossip.
"I knock around with a different set of teachers," she said. "I make a point of not sitting with him in the staffroom. It's not that he's a bad bloke, but I don't want to sit with my dad, really."
She follows the example of other teachers and calls him Mr Duffield during the school day, and Ian in departmental meetings. But the habits of a lifetime die hard.
"Occasionally, he passes me in the corridor and tells me to put my shoulders back. And he lends me dinner money, if I've forgotten it, and a warm top, if it's a cold day."
Her niece Laura Duffield, 17, also cites the supply of ready dinner money as one of the advantages to having relatives at school. Now in Year 12, she has been taught both by her grandfather and her aunt.
"I used to feel self-conscious about it," she said. "I'd worry that, if I was too loud, they'd tell my dad. If my granddad and aunt were really nasty, it would have been embarrassing. But everybody likes them, so it's all right."