All UK schools should be required to have a comprehensive sun safety policy to protect children from skin cancer, a charity campaigner says.
A recent survey of 1,000 parents, commissioned by MPs on the All-Party Parliamentary Group on Skin, found almost 40% of pupils have suffered sunburn while at school.
Richard Clifford of Skin Cancer UK says this is "entirely unacceptable" and wants mandatory sun rules for schools.
Current guidelines are not compulsory.
The call comes as parents of a 10-year-old girl who attends a school near Swansea complained their daughter had suffered severe sunburn at a sports day.
Pennard Primary School said parents could come in at lunch-time to apply cream, but that it would be impossible for teachers to apply suncream to the 200 children attending the school.
Advice from the National Institute for Health and Clinical Excellence (NICE) says schools should encourage children to seek shade whenever possible, particularly when out playing in the midday sun.
And it says sunscreen (of at least SPF 15 strength) should be applied liberally half an hour before and after going out in the sun and reapplied every two hours thereafter.
Nurseries and pre-schools have higher staff-child ratios to deal with such demands.
But what is not clear is exactly how schools are expected to deliver this.
Mr Clifford said: "Admittedly teachers cannot be expected to apply sunscreen due to simple time pressures.
"There is also the inevitable question regarding their concerns over child abuse and the strong advice they receive from local authority education departments and trade unions.
"However, there is no reason whatsoever why they should not supervise the application, perhaps with the assistance of the school nurse or indeed parents who attend on a pre-arranged rota system."
NICE suggests parents could provide sunscreen for their children who could, in turn, be taught how to apply it for themselves and their school-friends.
The guidelines also recommend schools run awareness campaigns to alert children to the harms of too much sun exposure.
But Mr Clifford says in reality too few do.
"Instead, it is left to the individual teacher to decide as to whether or not to introduce the subject of UV awareness. This is simply not good enough."
Cancer Research UK has produced information to help schools develop a sun safe policy, but so far only 621 English primary schools have registered to receive this sun protection policy pack.
Dr Claire Knight, health information officer at Cancer Research UK, said: "It's important that children are protected from sunburn while at school.
"Children have delicate skin and getting sunburnt as a child increases the risk of skin cancer later in life."
Prolonged over-exposure to the sun and episodes of sunburn under the age of 15 are major risk factors for skin cancer in later life.
The British Association of Dermatology estimates that four out of five skin cancer deaths are preventable.
Dr Knight said: "Cancer Research UK encourages schools to develop their own sun protection policy, and recommends that children should be allowed to bring sunscreen of at least SPF 15 with four or five stars into school and be supervised whilst applying it.
"Using shade and clothing is also crucial to any policy designed to promote the safe enjoyment of the sun."