The TES jobs pages tell a familiar story. Thousands of schools are trawling an ever-shrinking pool. “Wanted in January,” their advertisements say, “in this over-subscribed, dynamic and successful schoolI” The difference this year is the number of schools which, after “January”, add the significant phrase “or as soon as possible”. Usually it will be a secondary school with vacancies in a widening range of subjects: maths, science, technology and ICT, languages, PE, religious education and, increasingly, English. In the interim, the posts are being covered. Time-tables have been adjusted; supply teachers, part-timers, retirees, even, on occasion, the teacher whose retirement caused the vacancy, have been pressed into service. Somehow, the children are being taught. But these are expedients. There are vacancies to be filled “as soon as possible”.

Schools are cagey about the reasons for these gaps. It’s maternity leave, they say, or sickness, or long-postponed retirement; it’s a sudden growth in pupil numbers. And yes, they’re fully staffed. It’s just that they want the added commitment that comes with a permanent appointment. Off the record, they tell a different story. Too many supply teachers inappropriately qualified for the vacancies they are covering; too many of their own staff teaching outside their own subject areas.

Not that there is anything new in having to teach a subject that you didn’t train in or take a degree in, especially in the lower years of secondary school. “It’s been an issue for years and years,” says Nigel de Gruchy, general secretary of the National Association of Schoolmasters Union of Women Teachers. Schools have always had odd corners in their timetables - some English here, some French there - that couldn’t be covered by the specialist staff, and odd corners of specialist time elsewhere that would otherwise go unused. As long as there were no health and safety implications, it made sense to fit the corners together.

It’s the scale of the problem that’s new. Mr de Gruchy says his union’s figures back those of the Department for Education and Skills: 25 per cent of secondary lessons are taught by staff not trained in that subject. John Bangs, head of education at the National Union of Teachers, suspects the rate is even higher. “It’s a huge concern,” he says. Different, also, is the fact that this is happening at a time when subject knowledge and understanding is seen as the keystone of the teaching standards framework, and is fundamental to performance management and performance-related pay. Are teachers in this position disadvantaging their pupils? Are they disadvantaging themselves?

Not necessarily, say the heads who employ them. Good teachers benefit in career terms from having a second string to their bow; out-of-subject teaching widens their range and deepens their expertise. Besides classroom management is the really crucial skill. “Put a teacher who wants to teach with pupils motivated to learn,” says one head, “and anything is possible.”

That sounds like special pleading - but Jill Clough, tackling the challenging task of turning round the twice-failing school that is now the East Brighton College of Media Arts, insists it isn’t. “Far too many teachers have been led to think that content is all that matters,” she says. “It isn’t. Helping pupils to make sense of what they are learning is the most important thing, and we are most likely to do that if children have fewer, better teachers. In Years 7 and 8 we want specialists in learning, not in subjects. Ideally, all staff here teach outside their special subjects - and they become better teachers for it.”

Her colleagues say the approach is working. Head of year Adam Brazier, for example, has added maths and English in Years 7 to 9 to the geography he trained for, and is glad of that. “I learned the hard way,” he says. “Good mathematicians don’t always make good teachers. They don’t see what I have seen: that sudden click when the child who says ‘I can’t do maths’ finds that, with your help, she can. It gives me a real thrill when that happens. It’s better by far than interlocking spurs and hanging valleys.”

Teachers in other schools have similar tales. Pat Matthewson, for instance, trained to teach RE with subsidiary music, but has been teaching lower school history, geography and English (as well as RE) at Boston Spa comprehensive in Yorkshire for 16 years. “At the time it’s a lot of extra hassle, but you develop teaching skills you will use for the rest of your career. And I do think it’s better for the children.”

Emma Thornton was appointed to teach music at the same school, but was persuaded last year to take an English group as well. “It meant lots of extra work, coping with the planning and preparation, and because of my music I don’t have lunch times. But I had really good guidance from the head of department, and the kids did well, and I’m doing it again this year. I’m not sure that it’s a good career step, though. I want to be a head of music, not of English.”

Bev Hatton trained in history and taught it in her first post. After a career break to raise a family, she wanted to teach again but there was no local history post available. However, her old Worcestershire high school, where she’d also been a pupil, pleaded with her to come back “on supply”. That was in 1991, and she’s been there since, in temporary posts at first and then on a permanent appointment. In that time she has taught RE and English, some GCSE maths (“difficult at first from an O-level basis, but I soon found it easy and straightforward to follow”) and a bit of French. “No history, though.”

Her experience is typical. She is adaptable, capable, unflustered. “There’s only one rule to follow. As The Hitch hiker’s Guide to the Galaxy puts it: ‘Don’t panic’.” Above all, she is available. She is, as one head says, the Polyfilla of education, helping schools day in, day out, to hide the shortages and paper over the cracks. “Very often such teachers are women; almost always, they’re on temporary contracts.”

So you have to be careful about taking too rosy a view. Bev Hatton had the advantage of strong departments, good schemes of work. Asked to tackle RE, she enrolled on a distance learning course, and got a qualification - and confidence. “I enjoy crossing subject boundaries and seeing how other teachers teach. The downside is that there is a vast amount of extra preparation and, because my timetable changes, it’s often wasted. And though I have lots of support, I never really belong to any one. It can be lonely.”

But the Polyfilla factor means you tend not to hear about disasters. The probability is that most of the heads pleading for applicants for January “or sooner if possible” have something close to a disaster on their hands. Take, for example, the school in the north of England that had to advertise twice to get one applicant for the post of head of maths - and is still three mathematicians short in a department nominally of five. “We’re covering the gaps with supply teachers and, I’m afraid, with me,” says the head. “Frankly, the children deserve better.”

It’s an increasingly familiar pattern. In one of the largest local education authorities in the Midlands, the senior maths adviser calculates that only one high school in three is fully staffed with qualified mathematicians. The authority’s response is to target its numeracy support on what it euphemistically calls “the less experienced teacher”. It has two places for each school. The story is the same in the west of England. One school has two of its maths posts filled by “less experienced teachers”. One of them is a scientist, persuaded to stay on after his retirement.

The other is Sally. Family commitments mean Sally (not her real name) is tied to the area. She is a mature entrant and her subject - although she hasn’t taught it yet - is design technology. She has no maths; she had to resit her maths GCSE to qualify for her postgraduate certificate. She desperately needs a post to achieve her qualified teacher status, and when her school offered her a full-time maths post to GCSE standard, she reluctantly accepted.

“It was this, or a term on supply. I had to go for it.” Two weeks into the term, she has had “wonderful help from the department”. And the resit course she was forced to take has dissolved her own maths “block”. “I really loved that evening class.” She has been able to sit in on colleagues’ lessons; she has a huge file of teachers’ notes. “Apart from that, I’ve been too busy to feel anything - except, perhaps, that I’m being used.”

It’s a genuine problem. All schools need some teachers who can teach a second subject, and the growing emphasis on cross-curriculum studies such as PSHE and citizenship means that every teacher may need to do so. Properly handled (see panel), second-subject teaching should be part of your career development. In the current teacher shortage, though, it’s being taken too far. It’s a case of desperate circumstances needing desperate measures - not Polyfilla.

Meanwhile, take heart from Conn Iggulden’s story. Conn was head of English at St Gregory’s school in Kenton. Covering a history lesson for an absent colleague, he glanced idly at the pupils’ workbooks. They were about Julius Caesar. What a wonderful subject, he thought, for an epic novelI and slowly a synopsis took shape for a trilogy called Emperor. He sent it to an agent, and to his astonishment it sparked a bidding war. The upshot? An advance of pound;300,000 from HarperCollins. And a vacancy at St Gregory’s to be filled “as soon as possible”.

Teaching outside your subject?

* Don’t panic. Being capable and unflustered is part of being a good teacher.

* Insist on seeing the syllabus and the scheme of work. A strong department really helps.

* Find out what support is available: lesson plans, software, technical back-up, mentoring.

* Contact your LEA. What training can they offer, when?l Visit the TES website: www.tes.co.uk (www.tes.co.ukyour_subject) for links to free downloads of new ideas and new resources.

And if you still feel you’re being unfairly pressured, contact your union for advice.

* With the right support, your teaching skills will develop.