Pupils had to keep their coats and hats on at a school facing a cash squeeze as the head tried to save money on heating, the National Union of Teachers' conference was told.
Jo Yurky, founder of Fair Funding For All Schools, said pupils had gone without text books and teachers battled on without photocopiers.
She warned of the scale of funding cuts in some of England schools.
The government says school funding has been protected.
The mother and former parliamentary ombudsman, speaking at the union's annual conference in Cardiff, described how schools were "asking parents for regular payments to plug the funding shortfall - schools with £100,000, £200,000, £300,000 deficits begging parents for help".
"One school near me raised over £80,000 from its parent community - but it isn't enough," said Ms Yurky, who set up the parents' group to campaign over school funding.
"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 become a bit more careful about when they'll turn the heating on - to save money.
"That school can't afford to buy the text books the pupils need, but they also can't afford to photocopy them because that budget's been cut too.
"A primary school nearby had to cut a teaching post, a teaching assistant post, its music club and its science club - to save money."
Now a childminder in Haringey in north London, Ms Yurky said: "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.
"This is the state of things because there is a significant financial problem with our schools.
"The reason we are angry about it is because the government does not accept there is a problem."
Ms Yurky said another school, a grammar in north London, sent begging letters to ask for charitable donations.
"It's money they use to balance the books," she said.
"That school has also cut a history post, half an English post, and four subjects from its A-levels."
She said: "Of course parents want to support their schools, but there aren't many of us who have much money to spend at the moment.
"But we also think why is there a problem with money? The government says funding is protected. We would rather the government talked to the schools and sorted out the problem."
The Department for Education says that school budgets are being protected and that funding is at a record level.
"We recognise that schools are facing cost pressures, and we will continue to provide support to help them use their funding in the most cost effective ways, so that every pound of the investment we make in education has the greatest impact - and every child has the opportunity to go as far as their talents will take them," said an education department spokeswoman.