Teachers should refuse to apply sunscreen to pupils to avoid laying themselves open to allegations of abuse, the NASUWT has warned.
Pupils should be protected from skin cancer and sunburn by being kept indoors or in the shade during the current heatwave, Chris Keates, the union's general secretary, told The TES.
Ms Keates said teachers who were asked to apply suntan lotion to pupils should tell their headteacher that it is not part of their job.
Her comments follow warnings by cancer charities that schools need to develop policies to help protect children from the harmful effects of the sun.
Temperatures this week reached 32C?? in the South-east.
Ms Keates said: "Clearly, children have to be protected but we are extremely concerned about teachers taking responsibility for applying sunscreen to children in the present climate of false allegations.
"I am not saying that teachers should not care, but it is safer for teachers to question whether children need to spend time in the sun.
"It seems a very simple thing but it's one that could get teachers into enormous difficulties."
Advice to nurseries and primary schools given by the charity Cancer Research UK suggests that schools should obtain parental permission for teachers to apply suntan lotion to pupils.
But the NASUWT said that such permission would not be enough to protect teachers from abuse allegations.
Cancer Research UK recommends that schools should provide sunscreen of factor 15 or above free of charge to all pupils and ensure its regular use, especially around mid-day.
Outdoor activities should take place before 11am or after 3pm and pupils should be encouraged to wear wide-brimmed hats and sunglasses outside.
Morning break should be extended and lunchtimes shortened so that children spend less time outside under the mid-day sun, the charity said.