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The crisis of school funding in England is turning into a catastrophe, the largest teaching union has said.

Kevin Courtney, general secretary of the National Education Union, said schools were not absorbing budget cuts, instead many were cutting posts and increasing class sizes.

He highlighted research with his union suggesting many schools were covering classes with teaching assistants.

The government said core school funding was rising to £43.5bn by 2020.

Mr Courtney said: "Schools minister Nick Gibb would have us believe that schools have absorbed the cuts.

"We think that's absolutely wrong, schools have not absorbed these cuts and the cuts are going to continue over the lifetime of this government."

'Funding tight'

He said a generation of children were being made to shoulder the impact of the economic crisis.

"Whoever caused this economic crisis, it wasn't our five-year-olds in schools now," he said, adding that they had not even been born when Lehman Brothers collapsed at the start of the financial crisis in 2008.

Schools have been struggling as they face rising costs and growing pupil numbers.

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The survey of 900 teachers presented at the union's conference in Brighton also suggested schools were topping up budgets with parental contributions and by renting out buildings.

Ministers have insisted that more money is going into schools, although earlier this month the Education Secretary Damien Hinds did acknowledge that funding was "tight".

One in five of those questioned said their school had asked parents for money to help with school funding in the current academic year.

Nearly two thirds (64%) of secondary school staff polled and just over a third (35%) of those working in primaries said their school was renting out buildings.

Some 5.5% of those in secondaries and 3.5% of those in primaries said their school had accepted advertising on its premises.

A spokesman for the Department for Education said by 2020 schools would be getting 50% more per pupil in real terms than they got in 2000 - and that the introduction of the National Funding Formula would address historic funding disparities.