A new school year, but that same old problem - what to wear in the classroom? You want to be comfortable, but you don’t want to look like some elastic-waisted hangover from the 1960s. And while you know you need to look professional, you don’t want to end up looking like an undertaker’s assistant.

“Smart casual” is most teachers’ aim, but also one of the hardest looks to get right, according to Susan Bull, chief fashion adviser for John Lewis in London’s Oxford Street. So many teachers have approached her for advice about what to wear for work that she and her team are putting on a special seminar later this month.

John Lewis runs a free fashion advisory service at all its stores. You make an appointment and receive practical help picking out clothes that are right for you and your lifestyle. There’s no pressure to buy, and you don’t need a big clothes budget to do it. “People come to us because they know they need help, just as I’d go to someone if I was buying a computer,” says Ms Bull.

So what could John Lewis do for The TES’s two valiant teacher guinea pigs? Stevan Borthwick, 33, is a senior manager at Peckham Park primary school in the London borough of Southwark; his wife, Natasha Cameron, 29, is a newly qualified teacher of English starting her first job this week at Malory school, Bromley, in the London borough of Lewisham.

Stevan says his clothes have become smarter as he’s climbed the career ladder - he usually wears a tie to work, and always has a jacket with him. “It’s important to look right yourself if you’re trying to teach children to present themselves well.”

Chief men’s fashion adviser Geri Cox was impressed that Stevan knew his sizes, although he was half-an-inch out when it came to his shirt collar. She was also pleased about his willingness to try a wide selection of clothes. She persuaded him into some lilac and pink shirts, and found what he considered a “very loud” tie, with toning cuff links.

Natasha tends to opt for black trousers and white shirts, but Ms Bull put her into a variety of more colourful, striped shirts, and some dark grey trousers. “You suit fluid trousers,” she said, scrutinising Natasha’s body shape. “Shirts are good for you, especially if they’re a bit fitted.”

But she knew that teachers - particularly in secondary schools - must avoid tight or revealing clothes, and she understood that too sharp a look would not be right for someone working in a school with disadvantaged children. “You don’t want to put them off by going in looking like a banker.”

For women teachers who worked with very young children, she said, soft, tactile fabrics, and skirts and dresses could often be appropriate. “And pretty. Little ones love their teachers to look pretty.”

The three-piece black suit she picked out fitted Natasha perfectly, apart from the trousers, which could be taken up. Although at pound;265 it was expensive, Ms Bull pointed out that it was washable and would probably last twice as long as cheaper clothes. Plus, the waistcoat-jacket combination meant she could wear it in various ways, and for day and evening.

The advisory service at Oxford Street works out of comfortable private fitting rooms - grey for men, peachy for women - with good, large mirrors.

Watching while Stevan and Natasha tried clothes on, it was clear how rigid we all are about what is and isn’t “us”. Natasha looked stunning in a black and pink patterned top, but quickly tore it off, crying, “Ugh! Flowers!” So what was the couple’s verdict? “When she said I didn’t suit white, that was a bit of a shock,” says Natasha. “I think I’d now be inclined to go for brighter colours. It’s a good service. It takes away all that getting all your gear off, trying clothes on, taking them off, putting your own clothes back on, and going out to get more.”

“I grew to like what I’d got on, although I wouldn’t have picked it out myself,” says Stevan. “And I enjoyed doing it. I’d say to anyone give the service a go.

“You get to see which colours work and what various fabrics feel like. I think most men tend to head straight for what they think they like; then the danger is you never try anything new.”

Both agreed that the fashion advisers were friendly and practical. But with a young daughter to support, and little spare cash, they said the clothes were much more expensive than they would usually buy. “Unless, of course,” said Stevan thoughtfully, “it was something you really, really did take to. ”

John Lewis’s seminar for men and women teachers is on September 19 at 6.45pm at John Lewis, Oxford Street, near Oxford Circus tube station. Bookings: 020 7514 5302. Enquiries about the nationwide John Lewis fashion advisory consultations: 020 7629 7711