A quarter of schools surveyed by the union in England and Wales have been forced to close parts of their buildings already.
From January tighter regulations coming in as part of legislation to bring Britain in line with European standards, will require governors and local authorities to keep schools in 'efficient working order and good repair'.
The backlog of repairs was estimated at £4bn, but many schools built between 1950 and 1970 are reaching the end of their design life and the problem will become many times worse reports the guardian.
A special look is taken at Nottinghamshire CC's Parkdale primary where pupils are taught in portakabins and the paper reports that even cupboards are used for classes.
NUT general secretary, Doug McAvoy says:
'It's clear that teachers and pupils are exposed to daily physical and medical risks from unsafe and unhygenic buildings'.
The Guardian also reports the resignation of 18 governors of a Derbyshire CC school, Newbold Community school, over a £250,000 budget cut.