About 80 per cent of school kids aren't protected from the sun during lunch, according to research from Otago University.
It is the worst time of day to be exposed to harsh UV rays.
Auckland Primary Principals' Association Kevin Bush told TVNZ 1's Breakfast show that he was surprised by the findings.
He said it could be due to kids losing their hats and kids being unenthused about wearing them.
Mr Bush said most schools have a no-hat no-play policy but some schools do not have enough shaded areas.
"I think we've just got to make hats cool."
He said parents needed to make sure their kids were wearing hats outside school too.
The Otago study looked at 1300 children from eight Wellington schools.