School budget cuts are “endangering” efforts to improve education for poor pupils, according to a survey that found two-thirds of headteachers say they have cut staff to make ends meet.

The survey of 1,700 teachers and school leaders in England, published by the Sutton Trust educational charity, found that two out of every three secondary school heads said they had cut teacher numbers to save money, while primary school heads said they had also axed teaching assistant posts.

The survey conducted by the National Foundation for Education Research also found that many schools were using pupil premium funds to avoid deficits rather than their intended purpose of helping disadvantaged pupils.

“Our new polling adds to the growing evidence that the squeeze on school budgets is having a detrimental effect,” said Sir Peter Lampl, the Sutton Trust’s executive chair.

“Of particular concern is that schools are having to use funding for poorer pupils to plug gaps in their finances. Many are having to get rid of teachers to close these funding gaps and endangering efforts to improve opportunities for poorer young people.”

The charity, founded by Lampl to campaign on improving social mobility through education, was also concerned by evidence that schools were dropping class trips and outings to save money, while more than half said they had cut back spending on IT.

“Only a government with its fingers in its ears can continue to ignore the overwhelming evidence of the damage being done by real-terms cuts to school funding,” said Geoff Barton, general secretary of the Association of School and College Leaders.

“It is important that the public understands this situation will get worse unless the level of school funding is improved as a matter of urgency.”

Angela Rayner, shadow education secretary, said staff cuts would affect pupils with special needs. Photograph: Christopher Thomond/The Guardian

The survey comes after the Guardian’s recent reports that 1,000 state schools in England were using crowdfunding and online wishlists to encourage parents to buy books, pencils and other supplies for their schools. Another report found that teachers were having to take over as cleaners and lunch supervisors to cut costs.

Angela Rayner, shadow education secretary, warned that the staff cuts would affect pupils with special needs. “Teaching assistants often provide dedicated services for the pupils who most need extra support and these figures suggest they have been the first to suffer from the government’s cuts.”

The Sutton Trust said it was urging the government to address the financial uncertainty that schools are facing. It wants the upcoming spending review to take place as soon as possible to provide funding clarity for schools, as well as continued support for disadvantaged pupils through the pupil premium.

Pupil premium funds are worth about £1,000 a year for each pupil who has received free school meals, and are intended to be used to close attainment gaps between disadvantaged pupils and their better-off peers.

In response, the Department for Education (DfE) maintained that spending on schools was at record levels in England.

“There is more money going into our schools than ever before, and since 2017 we have given every local authority more money for every five- to 16-year-old in every school and made funding fairer across the country. There are more teachers in our schools than in 2010 and the number of teaching assistants has increased by a fifth between 2011-17,” a DfE spokesperson said.

But critics say the DfE’s figures fail to account for the substantial rise in pupil numbers since 2010, and overlook the cost increases in pension and national insurance contributions as well as teacher pay rises coming out of school budgets.

Analysis by the Institute for Fiscal Studies found that spending per pupil in England’s state schools fell by 8% in real terms between 2009-10 and 2017-18.

Meanwhile, a new poll of teachers by the NASUWT union found that 80% said they had been bullied by other staff members. In the majority of cases the bullying came from their senior manager, according to members who answered the union’s online poll.

“Bullying is destroying many teachers’ physical and mental health, and driving some teachers from their schools or the profession entirely,” said Chris Keates, NASUWT general secretary.