Councils in England have paid out at least £10m in compensation to people who developed illnesses because of asbestos in school buildings.
Figures obtained by BBC News show that in the past decade 32 councils have settled claims from former teachers, school staff or pupils.
The National Union of Teachers says up to 300 adults die each year because of exposure to asbestos while at school.
The government says it is investing £23bn to improve school buildings.
But campaigners warn that the presence of asbestos in schools continues to put pupils lives at risk.
“My mum Sue was a teacher for 30 years and her life was cut short because of this horrible material,” Lucie Stephens said.
“As she was dying she was really angry and concerned about the 900 children that she’d taught during her career. If my mum has been exposed to this deadly substance, how many of those children will have been exposed?”
Using the Freedom of Information Act, BBC Yorkshire has obtained figures from 135 councils in England, that show there are at least 12,600 council-run schools where asbestos is known to be present.
The number of actual schools that contain asbestos is likely to be higher as many have become academies and so are not included in the figures.
It is widely accepted that any school building built before the year 2000 is likely to contain some form of asbestos.
In the last five years local authorities have also recorded 99 instances of significant asbestos disturbances, where people have been put in danger because of potential exposure to the substance.
What is Asbestos?
Asbestos is a naturally occurring mineral that was commonly used as a building material because of its fireproof and insulating characteristics.
A complete ban on the use of the material was introduced in the UK in 1999.
If left undisturbed the material poses no risk to human beings.
However, if someone breathes in asbestos fibres, it can cause serious illnesses such as asbestosis and a type of lung cancer called mesothelioma.
Mesothelioma kills around 5,000 people each year. By the time it is diagnosed it is almost always fatal.
Dennis Law, from Sheffield, lost his wife Susan who died at the age of 64 from mesothelioma.
“The end of her life was horrific, she had ulcers and red blotches all over her body.
“For 20 years my wife was a teaching assistant and a dinner lady, and she shouldn’t have died because of where she worked”.
The BBC’s investigation also discovered there is no uniform approach to monitoring the presence of asbestos in schools in England.
Of those that responded, 13 councils said they held no information about which schools in their area contained the hazardous material.
Ten councils also refused to disclose information about the number of asbestos cases it had settled; meaning it is likely that more than £10m has been paid out in compensation to victims.
“This is a ticking time bomb because very few teachers and parents know that there is asbestos in schools. The very least we should do is make sure that this information is available to them,” said Rachel Reeves MP, chair of the Asbestos in Schools group.
Speaking to the BBC last month Education Secretary Justine Greening said arecent government review had provided schools with the latest advice in how to keep pupils and staff safe.
“In the overwhelming number of cases we do remove asbestos from schools, but in some cases our experts are telling us its better to leave the material in place. What our review had done though is given us a clear blueprint for how we tackle this issue going forward and that is what we are focussed on doing,” Ms Greening said.
A spokesperson for the Department for Education added: “The health and safety of children and staff in our schools is vital – that’s why we are investing £23bn in school buildings by 2021.
“This will help ensure asbestos is managed safely and that the amount in school buildings continues to reduce over time.”
Additional reporting by Nicola Hudson, Nicola Rees and Ruth Green.