Fires are typically started by school-age children

The biggest school insurer, Zurich Municipal, says the annual number of large school fires, causing more than �100,000 damage, has risen by 55%.

Arson attacks in England, Wales and Scotland are damaging or destroying an average of 20 schools each week.

The report into school arson also showed that the annual cost of repairing the damage had more than doubled in the past decade.

In particular, last year saw an increase in the number of very expensive school fires - with 42 attacks causing damage costing more than �5m to put right, up from 27 the previous year.

Disruption

But the total cost of school arson was lower for 2003 than for the previous year - reducing from �97m to �73m.

Arson has been a persistent problem in British schools - and there have been long-running campaigns to reduce the number of attacks.

These have highlighted that arson each year costs the equivalent of dozens of new schools and would allow thousands more teachers to be recruited.

There are other costs that are more difficult to quantify - such as the disruption to students preparing for exams, the destruction of pupil records or the loss of school work.

The Arson Prevention Bureau, backed by the insurance industry, says that arson of all kinds is a growing problem - with incidents having doubled since the early 1990s.

The bureau's chief executive, Jane Milne, said that it was difficult to say why there had been such an increase.

Video-games

But she said that in areas where there had been a concerted effort to cut arson, there had been a reduction in incidents - and she pointed to the midlands as an example of where arson was now less frequent.

Children who committed arson were sometimes not aware of the seriousness of their actions, she said, and it seemed that children, used to the restart button videogames, thought that the damage could be easily restored.

Previous research into the profile of a typical school arsonist has found that a large majority of fires are started by youngsters aged between seven and 17 years old.

Sprinkler system

But about a quarter of fires are believed to have been started by children who are aged seven and younger.

Zurich Municipal says that more schools should install sprinkler systems - particularly at a time when the government has launched a school re-building programme.

The Liberal Democrat education spokesperson, Phil Willis, has called for sprinklers to be compulsory in schools - saying that it would be a good long-term investment.

The Department for Education and Skills says that such fire prevention measures were decisions to be taken by individual schools and local education authorities.

It says putting a sprinkler system into all existing schools would cost �3bn.