Pupils are being taught in schools that are damp, have mould, or are simply not big enough, according to a survey of teachers.
It suggests that schools have had to take measures such as increasing class sizes and setting up temporary classrooms to cope with rising pupil numbers, while current buildings are beginning to crumble, or becoming unsuitable for staff and students.
The poll, conducted by the NASUWT teaching union, comes just weeks after official figures showed that secondary schools in England are facing a boom in pupils, with numbers set to rise by more than 600,000 over the next few years.
Overall, more than a third (37%) of the more than 1,200 NASUWT members polled rated the physical condition of the buildings they work in as poor, with a further 36% describing them as adequate, and the rest saying they are good or very good.
Around seven in 10 (71%) suggested that there are signs of leaks, damp or mould around their school, while just over half (55%) said the same about their classrooms.
One teacher told the union: "My classroom has thick black mould growing up the walls, the paint is peeling and the room smells very damp."
Another said: "The school is too small for the number of pupils. It is damp and cold - I have to have a dehumidifier running constantly in my room.
"There are patches of damp in the ceiling and the whole building needs painting inside and out. There is asbestos in the roof space."
NASUWT general secretary Chris Keates said: "Our children and young people deserve a learning environment which enhances their experience and provides them with the fit-for-purpose spaces and facilities they need and deserve."
Forecasts published by the Government show that around one in six secondaries (16.2%) and over a fifth (22.8%) of primaries are full, or in excess of capacity.
Secondary schools are now seeing an increase in numbers prompted by a spike in the birth rate in the early 2000s that is now being felt as pupils make their way through the education system.
Shadow education secretary Angela Rayner said: "It is completely unacceptable that under Theresa May's watch our children are being taught in schools that are damp and mouldy.
"Parents expect high standards for their children's education but first and foremost they want them to be safe. This Government, which is cutting capital funding for schools, is neglecting that basic duty."