Arson attacks on schools are increasingly taking place while pupils and staff are in the buildings, putting their lives at risk.
Up to a third of fires are now being started during the school day, according to the Government's draft guidance for headteachers.
"Received wisdom was that deliberate fires occur when the school is unoccupied so there is a low risk to life apart from to those lighting the fires," it says.
"However, this is changing with up to a third of fires starting during the school day when the risk to large numbers of pupils can be very high, with the younger pupils dependent on teachers and other staff for their safety."
Jane Milne, chief executive of the Arson Prevention Bureau, said its research also pointed to a rise in the number of daytime fires.
"The alarming rise in daytime school arson attacks is a tragedy waiting to happen," she said.
"Left unchecked it can surely be only a matter of time before these attacks bring loss of life or serious injury."
Around 70 per cent of the 1,300 fires which firefighters tackle at schools every year are believed to have been started deliberately.
Schools face a significantly higher risk than other public buildings such as theatres and cinemas because young arsonists regard targeting schools as a challenge.
Arson attacks on schools in England and Wales are estimated to cost more than pound;100 million a year in insurance claims, fire service costs, disruption and lost time.
The average bill is about pound;50,000 and the guidance says the number of attacks has now reached such a high level that they are putting intolerable strains on education budgets.
The largest fires still occur while schools are closed because arsonists can work undisturbed and avoid being caught, the report says.
But the document suggests that the growing frequency of fires during the school day means that staff need to be extra vigilant.
Daytime school fires occur most commonly in May, before the exam season begins, and during lunch breaks between 1pm and 2pm. Schools hit by daytime attacks in the past year include Caldicot comprehensive in south Wales, where two fires were started deliberately.
A pupil at the school had to call the emergency services on his mobile telephone when he was trapped in a blazing toilet block.
One head reported a fire being started at her school during a lesson covered by a supply teacher (see story on page 12).
Sylvia Moore said the teacher concerned was standing outside the classroom running a business from his mobile telephone, oblivious to the large hole being burned in the wall. No one was hurt, but the school no longer uses supply staff.
The new government guidance says: "Corridors and circulation routes might be considered as low-risk, but put school lockers, display boards and pupils together and the risk can easily increase."
In the past, schools have often decided against installing emergency lighting -except in public areas - because their facilities are used mostly during the day and pupils are familiar with their layout.
But the report says classrooms are increasingly being used out of normal hours by parents and other adults as a consequence of the drive to make all schools offer extended activities.
* Schools have until October 31 to give their views on Designing and Managing against the Risk of Fire in Schools, available at www.dfes.gov.ukconsultations