There is still the stench of smoke in the wrecked drama studio. For the arsonists, a pointless and petty act of vandalism helped while away a minute or two during a long weekend, but it is going to take most of Monday for Barry Davies, deputy head at Pentrehafod Comprehensive in Swansea, to sort out the mess, deal with the police and insurance assessors, and cope with the inevitable paperwork.

For him, it’s an uncomfortable reminder of the bad old days when the school didn’t have closed circuit television (CCTV). “Since we’ve had it installed, this sort of incident has more or less become a thing of the past,” he says.

So what went wrong? “Not the system itself,” he says. “We should have had another camera.”

He indicates exactly where he would like to have it located: on an exterior wall opposite the drama studio, high enough to include the broad sweep of buildings and grounds at the rear of the school. “If there had been one there, they probably would not have tried it. Or if they had, we would at least have had a good chance of being able to find out who they were.”

For headmaster Raymond Samples, the arson attempt is the exception that proves how successful CCTV, with other security measures, has proved to be in this 11-16 comprehensive in one of the tougher areas of town. Vulnerable windows have been fitted with grilles; the school is equipped throughout with intruder alarms; during the day, it has a security guard on duty at the main entrance. Joy riders, drunks and thieves have quickly realised that it is wiser to give the school a wide berth.

“We’ve had far fewer break-ins and far fewer broken windows. In the day time, we very rarely have anyone on the school premises who shouldn’t be here, ” he says.

Parents are delighted that security is being taken so seriously, and the 1,000 pupils assume that a school should adopt the same high-tech approach as the high street. Mr Samples is quick to dismiss any suggestions that the cameras herald a Big Brother attitude to discipline. “They are not there to spy on children,” he says, “but only to ensure their protection against unwanted visitors.”

It is no easy task. As well as the two main buildings, there are 19 classes housed in a tight cluster of demountables, those “temporary” blocks that somehow always manage to end up being permanent. They create a web of secluded alleyways, concealed corners and dark hideaways that can prove so attractive to bored kids with nothing to do of an evening.

“It would be impossible to keep the whole of an area like this under surveillance, but we can cover most of the points they’d be likely to enter, ” says Barry Davies.

Although it is unlikely that the local no-good boyos read The TES, it is sensible not to disclose where exactly the cameras are located or how many there are. It is safe, however, to reveal that some in fact are dummies - because it really is impossible to guess which.

The Vista VPM 3132 cameras, which are mounted exclusively on exterior walls, are used in conjunction with sensor-controlled lights so any night-time intruder within range of the lens is always fully illuminated. Ideally, the pictures should be transmitted to the control room of the local security firm where they would be constantly monitored, and action initiated the moment anything untoward happened. But, for a school, this is prohibitively expensive. Instead, they go no further than Mr Davies’s office.

Here a sequential switcher cuts regularly from one camera to the next. The pictures are displayed on a monitor and recorded on a VCR. The tapes are changed daily but, of course, they only need to be consulted if there has been an incident.

So on Monday morning Barry Davies is able to check on movements at the time of the arson by rapidly fast forwarding through the relevant section tape. He sees five youths enter the grounds and freezes the frame. As well as the picture, it displays the exact time that the recording was made. As it happens, this information is enough to establish that the youths, although trespassing, could not have been involved in the crime. If the extra camera had been in place, the real culprits would have been recorded and probably identified.

That might have ensured a little triumph for the cause of justice, but does it warrant the extra expense? Since its main value will inevitably be as a deterrent, it is always going to be impossible to calculate the actual savings that an investment in CCTV could make. But Raymond Samples is certain that it has been money well spent.

The initial outlay has, for instance, dramatically reduced the on-going expense of broken windows and the inconvenience they cause classes and teachers who have to be re-roomed. And then there’s always the serious consideration that a camera - even a dummy - could prevent something really awful happening. “You cannot put a price on children’s safety,” he says. “We have had to decide on cuts in other areas, but we didn’t even consider spending less on security. ”

Jayne Copues, manager of the Educational Premises Unit at the City and County of Swansea is also convinced of the cost-effectiveness of CCTV. The authority matches any money a school is prepared to spend on security with the result that many already have sophisticated systems in place, with others anxious to take advantage of any extra government money.

The price, depending on the lay-out of the school, can be as much as Pounds 40,000, but most schools can follow the example of Pentrehafod and be made significantly safer for as little as Pounds 5,000.

What three schools got for their money: * Pounds 700: One camera, one monitor * Pounds 5,700: Three cameras, one switcher, one VCR, one monitor * Pounds 18,000: 16 cameras, 20 external detectors, 20 external lights, five lighting controllers, one multiplexer (16-way), one time-lapse VCR, one monitor.

Pentrehafod Comprehensive was equipped by FAB Securities, 120 Gower Road, Swansea, SA2 9BT, tel: 01792 202718