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."
Just 36% agreed that classrooms and teaching spaces are large enough for the number of staff and pupils using them, while three in 10 (30%) agreed that communal spaces such as dining areas and halls were big enough.
The majority (70%) agreed that their school's outdoor space is large enough for the number of pupils that use it.
But less than half (42%) said that there are an adequate number of toilets and sinks for staff and pupils, with 47% agreeing that toilet facilities are in good condition.
Nearly half (48%) said that the number of pupils at their school has significantly increased in the last five years, with 16% saying numbers have decreased and the rest saying they have stayed roughly the same.
Of those that said pupil numbers have risen, 20% said the school has been extended, rebuilt or moved to a new site to cope with demand, while over seven in 10 (72%) said class sizes have become larger and just over a fifth (21%) said temporary classrooms have been created, or other areas of the school used.
One teacher told the NASUWT, which meets for its annual conference in Birmingham this weekend, that their classroom is "very small" with little storage space.
"Last year some children in my class of 30 had to crawl under tables to get to their seats," the union member said.
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."
A Department for Education spokesperson said: "We want all schools to be good quality and fit for purpose, and the fact is that thanks to our reforms and the hard work of teachers, nine out of ten schools are rated good or outstanding.
"We are investing £10 billion to maintain, improve or rebuild school buildings between 2016 - 2021, including £1.4 billion over the next year alone.
"This includes over £500m announced today for academy trusts and sixth form colleges to improve and expand their sites.
"We have also created 825,000 places since 2010 and despite rising pupil numbers, the average class size has seen little change.
"In fact, the average primary class size is 27. On top of building by councils, we have created 391 open free schools, 35 studio schools and 49 university technical colleges since 2010."