It is an overcast Thursday afternoon and Year 9 are reading out their creative-writing exercises. One pupil’s knowing description of smoking a cigarette draws a couple of smirks. “Very good, but I hope it’s not based on personal experience,” says Leigh Russell, the group’s English teacher.

But as soon as Joshua starts reading his meditation on dying, a hush falls over the room. “Death’s long arms cradle me like a tiny baby,” he says, finishing his piece. The class bursts into spontaneous applause.

It is perhaps no surprise that some pupils have become attracted to the macabre: their English teacher is now a celebrated crime writer. Mrs Russell’s first novel, Cut Short, was published only last year and has been nominated for the distinguished Crime Writers’ Association John Creasey (New Blood) Dagger, an award for first novels.

The psychological thriller follows a murder investigation conducted by Detective Inspector Geraldine Steel, a thirty-something workaholic whose passion and dedication to her job causes her personal life to suffer. Mrs Russell’s second book, Road Closed, published in June, follows DI Steel as she tries to solve her next big case. The third in the series, Dead End, is due to be published next summer.

Since her debut novel, Mrs Russell’s life has been full of book readings, signings, media appearances and visits to book groups. Despite this, she has managed to research and draft most of her fourth book already.

And she continues to work full-time as an English teacher at Aldenham School, a co-educational boarding school in Hertfordshire, where she has been a member of staff for the past 17 years.

In fact, she had planned to enjoy some down time during the past few years. Her two daughters have left home and she stood down from her position as head of learning at Aldenham. Then one morning, she went for a walk through the park to visit a pupil on work experience and a chance encounter gave her the inspiration for her first book.

“As I was coming up to a bend in the path a man approached me,” Mrs Russell recalls. “It was just me and him alone in the park. I had no idea who he was, but I just had this idea: what would I do if I walked on and saw a body in the bushes, and I had seen this man? I could become involved in this horrific incident just by being there at the time.”

Mrs Russell could not get the idea out of her mind and started to imagine a story about a serial killer, his victims and a race against time to stop him killing again. She started writing straight away. “Six weeks later, I had written a book,” she says.

That walk in the park not only inspired her first novel Cut Short, but also her main character Melanie Smith. She is an English teacher, one of only a few people to have seen the serial killer. She comes across him when she takes a walk through the park - to visit a pupil doing work experience.

There is a wide and varied cast of characters in the novel, but Melanie takes centre stage as we learn more about her life at home and at school. “Melanie is me,” admits Mrs Russell. “A grumpy, middle-aged teacher.” She insists that her portrayal of an irritating Year 9 class is an exaggeration, but Melanie’s frustrations at work are grounded in real experience.

So, too, is the book’s chatter between teenagers as they row with their parents, sneak out to parties and gossip about boys, all realistic details drawn from a lifetime of being around children.

Mrs Russell believes that teaching complements writing. “I’m endlessly fascinated by people,” she says. “In some ways, children are easier to observe as they don’t always conceal their feelings as well as adults do. As a teacher, you are constantly observing pupils, trying to find ways to engage them and encouraging them to work together, which means you think about them a lot.”

The reviews of her first two books have praised the psychologically astute portrayal of her characters, particularly the characterisation of the serial killer in Cut Short. Mrs Russell admits she became fascinated with what motivated him. Much of the book is dedicated to following his innermost thoughts and it quickly becomes apparent to the reader that he has learning difficulties.

Mrs Russell says his childlike nature made him an easy character to write. “I am glad to say I’ve never taught a pupil like Jim, but I find young people generally have very straightforward views on life and adopt a logical approach to things,” she says. “Once he adopts his completely insane attitude to women, he follows his own position to its logical conclusion.”

There are pitfalls to working as a writer and a teacher, Mrs Russell concedes. Teaching hours are long, usually without a proper lunch break, and there is always work to be done after working hours. On top of that, it can be difficult to stop worrying about particular pupils or incidents out of the classroom.

For Mrs Russell, writing three novels in as many years has involved grabbing time whenever she can - whether that is in the car on the way to work or during a particularly quiet free period. “I might be thinking about something throughout the week, when I’m stuck in traffic or something, and then I’ll write it really quickly on a Sunday because I’ve been mulling it over and writing a few notes,” she says. The holidays that come with working at an independent school also help. But she says the writing itself does not take very long - “it’s the thinking and the working it all out that takes time.”

The balance between a sociable profession and the more solitary pursuit of writing seems to suit Mrs Russell. “I can switch between the two very quickly,” she says. “If, on a lunchtime, I have finished marking, I may do a little bit of writing. It’s easier to write and then get back in the classroom than reading. It’s like changing channels.”

After studying English at Canterbury University in Kent, Mrs Russell started her teaching career in adult education and then moved on to supporting children with special educational needs. But she says she never had any aspirations to write herself.

Literature, on the other hand, has always featured heavily in her life. As a teacher, instilling a love of reading and building up pupils’ vocabulary have been her lifelong ambitions. “Children have so many things to distract them now - computers, TV, mobile phones - but it’s really important that they learn to enjoy reading,” says Mrs Russell. “I always say to them, if you can read and if you have good vocabulary you have access to all the information in the world and your life chances are vastly improved.”

When it comes to her own books, she has allowed a couple of pupils to read them, but it is not something she encourages. She prefers to keep her professions separate. “When I’m teaching, it’s not about me as an author - they are the ones who should be doing the work,” she says. But that does not always deter her pupils. “They ask me about it all the time - they are always trying to wind me up,” she laughs, rolling her eyes.

For the time being, crime writing is allowing the self-confessed nervy school teacher to let her imagination wander. “I’m a bit of a `what-if’ person,” she says. “I would like people when they are reading my books to be thinking that this could be happening in the house next door.” And with that, the bell rings. A new class swarms in, and Mrs Russell is back in front of the whiteboard.

THE BOOK CLUB: TEACHERS WHO HAVE BECOME WRITERS

• Dan Brown worked as an English teacher in the US until 1996, when he became a full-time writer. The Da Vinci Code was published in 2003; a film followed.
• Joanne Harris was a secondary French teacher in the north of England for 15 years. Her novels include Chocolat, which was made into a film in 2000.
• Michael Morpurgo, the former children’s laureate, was first inspired to write while working as a primary school teacher in Kent, when he made up stories for his pupils.
• Saci Lloyd, head of media studies at Newham Sixth Form College in east London, published The Carbon Diaries 2015 two years ago. The book was shortlisted for the Costa Book Awards. A bidding war for the film rights between Johnny Depp’s film company and the BBC followed.
• JK Rowling taught English in Portugal and then French in Edinburgh, where she finished writing her first Harry Potter book.
• Philip Pullman, author of His Dark Materials, worked as a middle-school teacher in Oxford in his 20s. He wrote school plays, one of which inspired his novel The Ruby in the Smoke.