Pupils had to wear coats and hats during lessons in January at one secondary school because the heating was turned off to save money, it has been claimed.
The same secondary also apparently could not afford to buy textbooks and children were banned from using the photocopier.
Tight budgets and funding concerns were highlighted at the National Union of Teachers (NUT) annual conference, which voted to back a one-day strike in England.
Any industrial action would take place before 31 August and union members would be consulted beforehand.
Teachers say Government spending cuts are pushing up their workloads and forcing staff out of the profession.
Campaigners say there is "a significant financial problem with our schools".
The conference in Cardiff heard parents at another school raised £80,000 to help patch up a £300,000 shortfall.
It came after they were sent begging letters by Latymer grammar in north London asking for help.
There are also claims a primary school had to cut a teaching post, a teaching assistant post, its music club and its science club to save money.
The Government has said it has protected the core schools budget in real terms since 2010, with school funding at its highest level on record at more than £40bn in 2016-17.
But campaigner Jo Yurky, from Haringey in north London, said: "In one secondary school near me, the children were taught for the first two weeks of January with their coats and hats on because they've had to be a bit more careful about when they turn the heating on, to save money.
"That school can't afford to buy textbooks that the pupils need, but they also can't afford to photocopy them because that budget's been cut too."
She went on: "If you look at the Department for Education's website, they tell schools how to make efficiencies. And so some schools, in trying to make efficiencies, they don't turn the heating on so much.
"It's not on over the school holidays, over Christmas, the children come back and the school's freezing.
"It's a school where they don't have funds to buy textbooks, they can't photocopy textbooks, they ask parents for money.
"Who would want their child to be in that classroom? Nobody. How does it help the child to learn? It doesn't."