Almost a third of primary schools and more than four out of ten secondaries are teaching pupils in temporary classrooms in the West Midlands.
In some cases the portable cabins, supposed to be for a limited period, have been in constant use for more than a decade.
And a union warned the number was likely to grow in the short term as a result of the Government's school rebuilding programme.
Out of a total of 1,155 primary schools in the region, 321 are having to teach pupils in temporary buildings.
Among secondaries, 123 out of 287 schools are using mobile outbuildings.
The Government claims it is putting "record funding" into improving the infrastructure of schools and a spokeswoman said the quality of modern facilities for temporary use provided a good environment.
But Bill Anderson, deputy general secretary of the Birmingham NUT, said: "We are always being told children only have one chance of education. No child should have to go through their primary and secondary education being taught, effectively, in huts."
In Birmingham, 86 out of 304 primary schools are using temporary classrooms - 28 per cent. At secondary level the corresponding figure is 32 out of 76 (42 per cent).
The authority, which is to undergo a #1 billion rebuild and refurbishment programme of all its secondary schools, claimed only 1.96 per cent of classrooms were "in a temporary state".
A Birmingham City Council spokesman added: "Thanks to securing recent major resources for the redevelopment of primary and secondary stock across the city, we are able to continue making significant progress in revitalising our school buildings."
Sandwell - the region's worst performing education authority and 145th out of 150 LEAs nationally - relies heavily on using temporary classrooms.
Out of 87 primary schools, 35 are teaching pupils in non-permanent buildings and 12 out of 19 secondaries use them.
Sandwell Council's cabinet member for schools and lifelong learning Coun Ian Jones (Lab Tipton) said: "Sandwell is actively pursuing resources to remove temporary classrooms wherever possible."
He added temporary classrooms could also be the sign of a popular and therefore grow-ing school.
In Worcestershire, which has long complained of under-funding compared with its metropolitan neighbours, 79 out of 186 primaries and 35 out of 57 secondaries use temporary structures.
The authority stressed mobile classrooms were used for a variety of reasons including catering for bulges in roll numbers or during refurbishment of existing school buildings.
Coun Liz Eyre, Worcester-shire's cabinet member for
children's services, said: "The issue around mobiles is not as clear as it seems. For a start, new mobiles are not like the ghastly buildings of yesteryear and I have seen some superb maths, art, cookery, technology provision delivered from mobiles around the county.
"It's about educational outcomes - so we shouldn't get too hung up on physical structures."
The Government has launched a #10 billion drive to modernise all of Britain's secondary schools under the Building Schools for the Future programme.
A spokeswoman for the Department for Education and Skills said: "Progress is being made year-by-year in improving the quality of the school building stock. Modern, high-quality mobile or demountable classrooms provide a good environment for teaching and learning where there is short-term need."
But Liberal Democrat shadow Education Secretary Sarah Teather said: "It's really appalling that so many schools in the West Midlands are still having to make do with temporary classrooms.
"Shabby, draughty temporary classrooms are no place to learn, they do not promote the positive attitudes and high aspirations we want to instil in our children."
Conservative shadow Schools Minister Nick Gibb, added: "No one minds a temporary classroom if it is helping a good school to expand but when nearly half the schools are in temporary accommodation that can only damage standards."