Caitlin Robinson is cautious of sun exposure after her mother died from a malignant melanoma but when she took a bottle of suncream to school during last week's heatwave she was prevented from applying it.
Health and safety regulations ban pupils from applying the cream themselves in case it comes into contact with a child who is allergic to it.
But teachers are not permitted to apply it because of concerns over personal contact and "potential pitfalls" such as applying the wrong lotion.
Caitlin, who is fair-skinned, came home from school suffering sunburn.
Her stepmother Karen Robinson said: "The school refused to allow my stepdaughter to apply suncream to herself during the day. Although I had applied suncream before the school day she still got burned.
"I explained that her mother died of skin cancer and she was of greater risk. I also explained that suncream does not last all day, even the ones that are for all-day application, due to being sweated off and rubbed off on clothing.
"I was not asking for the school to apply the cream, just that they allowed her to apply her own cream to herself. I really think it's disgusting.
"Caitlin was more than able to apply her own suncream and had done for many years, but they said it was for health and safety reasons because if another child has an allergy to suncream and it then drips on to that child or they pass the cream around."
The dispute at Perry Wood Primary School in Worcester was resolved when the school agreed to allow Caitlin to put suncream on under supervision.
Angela Beddow, the headmistress, said: "Once we became aware of the parent's concerns, we responded immediately by offering several solutions to resolve the situation, which has included putting a special care plan in place.
"This will enable the pupil in question to apply suncream under the supervision of a member of staff."
Katy Scammell, SunSmart campaign manager at Cancer Research UK, said: "It's important that children's skin is protected."
A spokesman for Worcestershire County Council backed up the regulations, saying the advice given to schools in hot weather is that staff should not apply sunscreen to pupils, even if it is requested by parents.