White asbestos was banned in 1999.
The NUT puts the UK-wide figure at 86%, based on a Freedom of Information request to local authorities.
Symptoms of the cancer generally take 30 to 40 years to develop. Once diagnosed, however, most people can expect to live between just 12 and 21 months.
Jenny was diagnosed in May 2013 and is hoping her chemotherapy has slowed the cancer’s progress. But her husband, Bromley, said they were “on borrowed time”.
“We’re looking ahead maybe six to eight weeks, maybe more,” he said, regarding the couple’s ability to make plans for the future.
Between 2003 and 2012, 224 people in Britain whose last occupation was recorded as “teaching professional” died of the disease.
The rate of deaths is rising, but only in line with overall mesothelioma deaths, which amounted to 21,957 during this same period.
Jenny said she did not have any hopes for the future but tried to remain positive where possible.
“Sometimes I cry because I just feel it goes on and on and on, but I’m fairly upbeat,” she said.
She has been given compensation by the local authority, which settled her claim with no admission of liability.
There are now strict government regulations on how to manage asbestos and monitor its condition in schools, but an NUT online survey – based on 201 responses – suggested 44% of teachers had not been told whether their school contained the substance.
David Martin, 70, from Mansfield, was diagnosed with mesothelioma three years ago.
It came as a shock, as his former school “was very careful, and had a plan of where the asbestos was located”.
Last year, he was given compensation by the local authority, with liability admitted over his exposure to asbestos while working as a school technician.
He had also taught woodwork and design at the school for 20 years.
“If it was anything to do with moving asbestos or working with asbestos, then specialist firms were brought in to remove it professionally,” he said.
“So I never thought I was really being exposed until after being diagnosed.
“I wasn’t aware that by banging doors that could have disturbed some of the asbestos fibres, which I’m now told it could have done.
“I put a projector and a screen in every classroom in the school… it could have been then that I was exposed.”
David described asbestos in schools as “a time-bomb waiting to explode”, and his main fear is for pupils.
“Children will be children,” he said.
“They will knock, tap, kick balls – no matter what signs you put up,” he says.
There are no statistics to suggest how many people might have developed mesothelioma due to asbestos exposure as a pupil, but the Committee on Carcinogenicity suggests a child first exposed to asbestos aged five has a lifetime risk of developing the cancer about five times greater than that of an adult first exposed aged 30.
A government review of asbestos in schools in March 2015 concluded if it was undamaged and managed safely, it did not pose a significant risk and it was safer to keep it in place.
The government said it would continue to develop more targeted guidance on asbestos management in schools and, where appropriate, fund its removal.
A Department of Education spokesperson said: “Billions has been invested to improve the condition of the school estate, with further significant investment to come over this Parliament. This funding will help to ensure asbestos is managed safely and that the amount in school buildings continues to reduce over time.”
But NUT general secretary Christine Blower said the government had no “long-term strategy” and there was “still no [government] recognition that asbestos is a serious problem for schools”.
Prof Julian Peto, of the London School of Hygiene and Tropical Medicine, said: “Because the levels of asbestos are [often] so low in schools, actually ripping it out could be more dangerous”.
“It’s a very nasty problem to get sorted. And for young people, the risk of dying of mesothelioma is more like one in 10,000, as there’s a disconnect between those being diagnosed now and the exposure to the children of today.”
He added that, were it possible, removing all asbestos from the environment would prevent “at most 25 deaths a year” – and only a fraction of these from exposure in schools.
“So trying to remove it from all schools could actually increase the number of deaths.”