This story begins several years ago in an Essex bar, where two men who have never met before are ordering a drink. One is David Black, a former sociology lecturer who runs a company supplying part-time teachers of shortage subjects to schools all over Wales. The other is Steve Moorhouse, a Southampton college lecturer who has invented a video-conferencing system adapted for use in schools.

Both have travelled to Essex to speak at a distance-learning conference the following day. But it is that night in the bar that the real work is done. For why, it suddenly strikes Messrs Black and Moorhouse, should anyone send teachers trekking to schools from Monmouth to Anglesey, when with some high-tech kit and a decent coursebook the same staff could teach the same courses from the comfort of their own homes?

And that, four years on, is exactly what Moorhouse-Black does. In September 1998 it signed up a dozen customer schools; today it has 150, predicted to rise to 250 by the end of this year. More than 1,000 state school pupils will be taught by Moorhouse-Black tutors from this September - all of them using just a coursebook, a telephone line and a monitor.

The service is aimed principally at schools that want to offer a sixth-form subject but cannot, or will not, employ a teacher to teach it. Under-16s who need a Latin teacher are also being catered for, and this age group may grow in other subjects.

For pound;2,850, Moorhouse-Black will supply a qualified tutor to give a weekly one-hour videoconference lesson to five AS-level students for the duration of the course. The tutor will set and mark all work, which is based on a self-study pack supplied to each student. Extra students cost pound;450 per head. For pound;4,125 plus VAT the company will also supply a video-conferencing system, replaceable within 24 hours if it breaks down. All the school has to do (apart from pay) is ensure that students turn up to their lessons, spend at least three hours a week on self-study, and email their assignments directly to the tutor, or hand them in to the school’s link teacher to send on.

Using this system, a school can boost its sixth-form curriculum by offering accounting, citizenship, critical thinking, electronics, film studies, government and politics, Latin, law, philosophy, psychology, sociology, and - from September - biology, chemistry, maths and physics.

This step into the mainstream curriculum is in response to requests from schools, says David Black. They have seen results from the video-conferencing method at least as good as, and in some cases better than, classroom-taught subjects. Miniature lab equipment and reagents (which Moorhouse-Black will also supply) make science practicals possible without a laboratory, although schools will need technician support for self-study periods, and the science courses will cost more: pound;3,420 for five students doing physics or chemistry, plus pound;540 for each extra student.

“We have provided subjects that schools traditionally have difficulty teaching from their own staffing establishment,” says Mr Black. “That’s now true of A-level physics and chemistry. We’re not suggesting every subject can be taught this way, nor that every student should be taught this way. But schools come to us and ask us to run courses.

“Some ask, ‘can you take our kids who want to do separate science GCSEs?’

Or, ‘our maths teacher is pregnant; can you teach the top set?’ Or, ‘can you teach the group that’s skipping GCSE to go straight to AS-level?’ I think we will end up moving into pre-16.”

The lessons

On the monitor, Clare Anderson’s speech and gestures, as she explains the subtleties of criminal responsibility, are metallic and jerky - like old astronauts moonwalking on film. That doesn’t deter her seven A-level law students at Temple Moor high school in Leeds from scribbling down notes as she gives them their weekly law lesson.

Earlier that day, the Year 13 students sat together in Temple Moor’s sixth-form work room, going through the coursebook notes on their next topic. So when 11am arrives and they move to the school’s video-conferencing room - a tiny classroom, back wall painted blue to reduce glare, over-exuberant Year 7 German class expelled from next door - they are ready to answer Ms Anderson’s questions as she checks what they have understood.

Temple Moor began offering law and psychology A-levels via video-conferencing four years ago, after sixth-form head Barbara Owen met a teacher from another school who was using Moorhouse-Black. Although the school now holds on to students who might have gone elsewhere, it refuses to allow any to do more than one subject by distance learning; it also screens some out as not possessing the self-discipline required.

Mrs Owen is the link between tutor and pupils. “I don’t sit in the lesson or the work room with them, but they know I check on them,” she says. “I get an attendance sheet each week from Clare, so I know if anyone’s not there. I get emails and a referral sheet if anyone’s behind with work, and I chase them. They send off a piece of work to Clare every week, and they have to give me a copy of it.”

Back in the video room Ms Anderson is explaining reasonable defences for criminal behaviour. A brief student digression on the subject of diabetic grannies is short-lived, as is a more relevant discussion on penal policy.

The informal, spontaneous debate that characterises the normal A-level class is impossible in this setting, says Ms Anderson, who is based in Newport, South Wales. “I have to stop discussions quickly because I can’t get involved in them and because we won’t get through all the material if we slow down.”

The students return to their note-taking, following Ms Anderson’s exposition in their coursebook and checking examples in the law textbooks, which the school provides. It is certainly an effective teaching method in terms of exam results - As and Bs were the standard grades at AS-level in this group last year, a higher average than for other subjects in Temple Moor’s open-access sixth form.

Initially, students were anxious about the lack of everyday contact, of being unable to catch their teacher in the corridor or outside the staffroom with a query. Moorhouse-Black provides a fax helpline and students have access to tutors’ email addresses and, in some cases, home telephone numbers, although only one Temple Moor student has used them.

Instead, say the students, if they don’t understand the course notes the first time, they read them again. “Not having a teacher there increases your self-motivation,” says Sue Stephenson, 18. “In other lessons you waste time; this is organised and intense, and it’s made us more like that in our other subjects. You make sure you come in because you know the teacher’s not going to go off in the middle of the lesson to do some photocopying.”

The teachers Giles Dawson nips upstairs in his Victorian house in Sydenham, south London, just in time to see the yellow warning light come on on the computer screen in the corner of his tiny study. His Year 10 Latin group in South Bromsgrove community high school in Worcestershire - one of 19 groups he teaches - is ready to start.

His four pupils sound muffled, until one removes her bag from the table, where it is blocking the microphone. They start work on deponent participles. Twenty minutes later, alarmingly, the screen goes blank. Mr Dawson shuffles the mouse and his class reappears.

His video equipment, like the ISDN phone line that serves it, was installed by Moorhouse-Black when he began tutoring for them 18 months ago. A former prep school deputy head and private tutor who had never taught in the state sector before joining Moorhouse-Black, he answered a newspaper advert unaware that video-conferencing was involved.

Most Moorhouse-Black teachers have tutored privately or taught in further education. They attend initial and in-service courses run by the company; they are also inspected “virtually” by Ofsted.

Their pay is not huge - Mr Dawson earns pound;20,750 a year for a 35-hour week that includes 19 hours of face-to-screen teaching. There is a lot of marking - in theory Clare Anderson marks an essay a week for each of her 50-plus students, although in practice “some students just turn up for the hour lesson and ignore you the rest of the week”. Ms Anderson works nine to five; but because Latin is an extra-curricular option in most of Giles Dawson’s schools, his lessons are clustered in lunchtimes and twilight periods.

As Mr Dawson ploughs through the ablative absolute and launches into Pliny, the South Bromsgrove students have their foreheads in their hands. This is hard work. They cover the GCSE in one year, with a high pass rate. He knows them not only through the screen but also because, like all Moorhouse-Black tutors, he visits them once a term in the flesh. “When I go into schools, I always look at noticeboards and newsletters, to try to find a link with my pupils. It’s strange; you are establishing a presence in each of these schools, even if it’s only for an hour a week.”

That presence is fairly tenuous, says Clare Anderson, who has taught law for Moorhouse-Black for five years and now supervises other tutors. Although Temple Moor is always welcoming, she says:“When you go into a school the staffroom can be quite hostile. They don’t really see us as teachers; all the link tutors are pretty amenable, perhaps because it’s usually been their idea to use the system, but other teachers can be mistrustful. Often I don’t even get a cup of coffee.”

South Bromsgrove head Phil McTague has no doubt that under-16s cope as well as older pupils learning this way. “It hasn’t been without its moments of pain as students move from being spoon-fed to a more independent regime,” he says, “and it would be more difficult in a mainstream class. But students gain so much from the more focused questioning.”

Giles Dawson agrees, although he admits it is not easy for the teachers. “It’s different from being in the classroom,” he says. “It’s a particular sort of intensity, partly due to the solitude. With a language you have to make it more like a conversation lesson, only done at a distance. The one thing that really helps is a big screen: one of my schools has one and I’m aware when I’m teaching them that I’m quite a dominant presence in the room.”

Moorhouse-Black: tel, 01248 679 025; www.moorhouse-black.com