Most state school buildings in South East England contain asbestos, an investigation has revealed.
Freedom of information requests by the BBC's Inside Out programme revealed the potentially-deadly material remains in more than 90% of schools.
Kent, Medway, Sussex, Brighton and Surrey councils all revealed a high proportion of properties affected.
The Health and Safety Executive (HSE) said it would be dangerous to remove asbestos sealed inside buildings.
But the National Union of Teachers (NUT) has called for all asbestos to be removed from all schools.
John Walder, secretary of the NUT Kent branch, said: "The NUT's view is quite clear, we think that the solution is the complete removal of asbestos from all the working spaces.
"As long as local authorities do not remove asbestos from a site, there will be a risk of fibres getting into the atmosphere and getting into people's lungs."
After WWII, the majority of state schools in the UK were built using asbestos and most have not yet been replaced.
Many of the schools in the South East have the least harmful white chrysotite asbestos, which was banned in 1999, and the more harmful amosite 'brown asbestos', banned in 1985.
A minority of schools also have the most dangerous blue crocidolite asbestos.
There is some form of asbestos in 185 of the 195 schools in East Sussex, 75 of the 77 schools in Brighton and in 271 of the 286 schools in West Sussex.
In Kent, 554 of the 599 schools have asbestos, with 60 buildings containing the crocidolite type, and 111 of the 116 in Medway are affected.
Surrey, which has 333 schools, has 303 properties with asbestos, 186 with the amosite type.
Dr Robin Howie, an independent asbestos consultant, said the number of teachers dying of asbestos-related diseases in the UK had risen from about one every two years to more than five a year.
He said:"We are looking at a substantially higher number of mesothelioma deaths in teachers than we would expect.
"What it means is that teacher mesotheliomas are important because they are the tip of the iceberg. And that iceberg are the mesotheliomas in children."
Elizabeth Bradford, a special needs teacher from Bromley in South East London, was interviewed by Inside Out before she died of mesothelioma two months ago.
Mrs Bradford, 70, who had worked in a classroom lined with asbestos, said: "I keep wondering about those children. I'd like to know if there have been any repercussions to those children but how do you know.
"I love my job, but it's quite shocking to think I paid quite heavily for a job I enjoyed."
Her son Matthew said he believed she inhaled tiny amounts of asbestos each time she removed a drawing pin from her classroom wall.
"I'm very angry about it," he said. "I feel people need to know about it and it needs to be taken seriously because it's a killer."
Schools where exposed asbestos is found are closed or cordoned off while the material is disposed of.
Rosalind Roberts, head of public services at the HSE, said: "If it is there and it is sealed in place then it represents no health risk to those using the building.
"It only represents a health risk if somebody comes along and drills into it and makes the fibres airborne and people breathe them in."
She added: "It would be a bigger danger to those in the schools for it to be removed."