Lucy Marsh, 77, died from the lung cancer mesothelioma, caused by exposure to asbestos.
A coroner said that in her 50-year teaching career Mrs Marsh had been exposed to brown asbestos, which caused cancer of the lung linings. The disease, which is incurable, is more common in industrial workers, such as plumbers or car mechanics.
Powys coroner Geraint Williams said: "I have not been provided with details of the schools where Mrs Marsh taught. But in many of our older school buildings brown asbestos was, and still is, present.
"Mesothelioma is a cancer known to be caused by exposure to brown asbestos.
Mrs Marsh contracted the cancer over a long working career."
A verdict of death from natural causes was recorded at the Brecon inquest on Mrs Marsh, the widow of a retired headteacher, from Painscastle, near Builth Wells.
Powys county council said it had no record of Mrs Marsh having worked in any of its schools.
Gethin Lewis, secretary of the National Union of Teachers Cymru, said it would investigate the case in an effort to pinpoint how Mrs Marsh could have contracted the cancer.
Mr Lewis said: "It is very worrying. We have a number of concerns about asbestos present in schools, mostly as insulation around boilers and in heating and roof tiles.
"Much of it is now coming to the fore due to work taking place in old schools, and areas are having to be isolated as the asbestos is removed safely. We will be carrying out an investigation to ensure that present teachers are safe from any future risks."
A Welsh Assembly Government spokesperson said: "This is a health and safety matter for schools and LEAs. The Assembly government does not have a specific budget for removing asbestos from schools, although funding for investment in buildings can be used to address safety issues."
A TES Cymru survey last week revealed Welsh schools faced a pound;785 million backlog of repairs, with some councils only able to afford essential safety work.
Last year, the NUT secured pound;135,237 from Greenwich council for the family of another teacher with mesothelioma, which usually takes many years to develop.
An inquest found that Shirley Gibson, 37, died in 1993 of an industrial injury, probably arising from her work at Plumstead Manor school in the south-east London borough. Asbestos was removed from 21 rooms in 1993 after staff refused to work there.
And in 2002, Fareham college in Hampshire was fined pound;23,000 plus Pounds 18,000 costs for exposing staff to asbestos. Public-sector union Unison had been campaigning for 15 years to have the material removed.
The Health and Safety Executive decided to prosecute after college staff complained they had been exposed to the deadly dust from 1993 to 1999.
Asbestos-related diseases kill around 3,500 people in Britain each year, a toll which is rising.