Four in five state schools in England will be financially worse off next year than they were in 2015 despite promises by Boris Johnson’s government of a multibillion-pound funding boost, according to research by teachers’ unions.

The School Cuts coalition of six unions, which spearheaded a national campaign for more funding in schools, has conducted an analysis of recent government announcements which it says shows that more than 80% of schools will have less funding per pupil in real terms in 2020 than they did in 2015.

The government has pledged to invest an extra £7.1bn in schools in England over the next three years up to 2022-23, including an increase of £2.6bn to the core schools budget in 2020-21, but the unions say there will still be a shortfall of £2.5bn in the coming year after years of devastating cuts.

Geoff Barton, the general secretary of the Association of School and College Leaders (ASCL), said: “After years of denying that there is a school funding crisis, the government has finally done the right thing by investing desperately needed extra money into our beleaguered education system. But analysis by the School Cuts coalition shows the additional funding is not enough to repair the damage that has been done to our schools and colleges and that further investment is required. We are not being churlish, we are just stating the facts. The funding crisis is not over.”

According to School Cuts, children in almost all local authorities in England will lose out and schools with the highest levels of deprivation will be worst affected despite the government’s funding injection.

The campaigning coalition group, which is made up of ASCL, the National Education Union (NEU), the National Association of Head Teachers, GMB, Unison and Unite, says 91% of schools face real-terms cuts compared with 2015. Additional government funding will reduce that figureto about 80% in April 2020.

Education for the 16 to 19 age group, which has been particularly badly hit by austerity in recent years, will still face a shortfall of £1.1bn, despite the government’s promises on additional funding, School Cuts claims. Even with the additional £700m promised for children with special educational needs and disabilities, campaigners say that high needs block funding, which supports the most vulnerable children and young people, will still be £1.5bn short.

Kevin Courtney, the NEU joint general secretary, said: “Johnson has made lots of empty promises on school funding – but his numbers don’t add up. The latest funding announcement falls well short of settling the shortfall for every child. And crucially it fails to reverse the cuts schools have suffered since 2015.”

John Richards, the head of Unison’s education division, said: “Schools are so cash starved that staff are buying equipment like pens and stationery with their own money. Valuable teaching assistants are also being axed by schools as they struggle to balance budgets. The government keeps promising resources but schools need money now.”

Luke Sibieta, a research fellow at the Institute for Fiscal Studies, said: “Total school spending per pupil has fallen by 8% after inflation since 2009 and by 5% since 2015.

“The government has committed to extra funding of £4.3bn per year in today’s prices, which will be enough to reverse cuts on average. However, that won’t fully come in until 2022. It’s therefore unsurprising to see analysis showing that most schools will have lower budgets in real-terms next year as compared with 2015.”

The Department for Education said the government had announced the biggest funding boost for schools in a decade which would give every school more money for every child. “We are investing a total of £14bn more in schools over the next three years to 2022-23,” a spokesperson said.

“This means all secondary schools will receive a minimum of at least £5,000 per pupil next year while all primary schools will get a minimum of at least £4,000 from 2021-22 – with the biggest increases going to the schools that need it most.”