Up to a third of fires are now being started during the school day, draft guidance from the Government for headteachers reveals.
Around 70 per cent of the 1,300 fires which fire crews tackle at schools each year are believed to have been started deliberately.
Schools face a significantly higher risk than other public buildings because young arsonists see them 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 pound;50,000, and the guidance says the number of attacks has reached such a high level it is putting intolerable strains on education budgets.
The guidance suggests that the growing frequency of fires in school hours means that staff should be especially vigilant. The most common times are in May, before exams, and in lunch breaks between 1pm and 2pm.
Schools hit by daytime arson attacks in the last year include Caldicot comprehensive in south Wales, where two deliberate fires were started. One pupil at the school was forced to call for help on his mobile, as he was trapped.
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