Other issues which could see schools spending too much on gas and electricity include a lack of loft insulation (16 per cent) and no insulation on pipes or valves (35 per cent).
npower claims that schools could save an average of £2,000 per year with energy improvements to make the building more efficient.
Clare McDougall, head of npower's Climate Cops programme, said the firm is working with schools to help them save money and energy.
She added: "With the government and opposition parties discussing potential cut backs in education budgets, adopting basic energy saving measures seems an obvious starting point."
Nebusiness.co.uk recently claimed that installing solar panels could cut a building's energy bills by up to 40 per cent.
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.