An alliance of headteachers, school governors, councils and unions are to lobby the chancellor for a further £5.5bn a year to help schools in England to avoid financial difficulties and cuts.

The coalition, involving almost every significant organisation involved in state schools – including the National Governors Association and a bipartisan group representing both Conservative and Labour-led councils – is writing to Rishi Sunak before next week’s budget seeking additional funds, with a focus on “woefully underfunded” provision for pupils with special education needs and disabilities (Send).

“Nurseries, schools and colleges are all struggling to meet growing costs and demands, with many having to reduce what they offer in order to meet their budgets,” the group says in a letter to Sunak and Robert Halfon, the Conservative MP who chairs parliament’s education select committee.

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A string of local authorities in England have been told by the government to drain their school budgets to help pay for their special needs obligations.

Sabrina Hobbs, the principal of Severndale specialist academy in Shrewsbury, one of the largest special schools in England, with children aged from two to 19, said any further budget cuts could imperil staff and students.

“The squeeze on special schools will soon begin to impact on the most vulnerable children within our communities as it will become impossible to accommodate their needs as resources and staffing are stripped right back,” Hobbs said.

“Leadership has been cut, management has been cut and curriculum opportunities have been cut. There is nothing else without risking health and safety and quality of life.”

Headteachers who back the lobbying say they are still being forced to reduce staff numbers and make cuts to essential provision, despite the government’s promises of an additional £7.1bn in school funding by 2022-23.

“Much of that extra funding will be swallowed up on rising costs including increased and unfunded pay rises that have been introduced. This jeopardises the promised real-terms increased budgets for the schools and pupils themselves,” the letter says.

The coalition says that once inflation and increases to teacher pay are taken into account, more than four out of five schools will be worse off this year than five years ago. Sixth forms and colleges will continue to be hardest hit.

James McInnes, a Conservative councillor who heads the f40 group representing the local authorities with the lowest school funding in England, said: “Although we very much welcome the government’s investment, our analysis is that £12.6bn was needed across the country, so that falls short by £5.5bn.

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“So we need to work together as a coalition of unions, local authorities, teachers and parents, and say that while we welcome the extra funding, we also need to keep the pressure on central government so they understand that’s not the end of the story, that additional funding is still required.”

McInnes, Devon county council’s cabinet member for children’s services and schools, said special needs funding requirements were causing budget deficits in councils around the country.

“In Devon in the current financial year, we’re £21.5m overspent in the high needs block of the dedicated schools grant – that’s mainly special education needs and disabilities. Next year if there aren’t any changes, we’re projected to overspend about £45m.

“We’re just one authority. Nearly every local education authority across the country is overspending in high needs, to various extents. So we need more money, but it’s not only about money, it’s about how [the government’s special education needs reforms] have been working.”

The Department for Education said: “This government wants to build an education system that boosts productivity, improves social mobility and equips children and adults with the skills and knowledge they need to succeed in the bright future that lies ahead.

“That is why we are levelling up funding to ensure all schools have the right investment to deliver an outstanding education. On top of this, we are increasing high needs funding for local authorities by £780m next year, as well as £400m cash boost for 16-19 funding.”

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The letter to the chancellor will be backed by the main teaching and school leadership organisations including the National Association of Head Teachers, the Association of School and College Leaders and the National Education Union.

The coalition includes Worth Less?, a group of headteachers who have been agitating for additional funding.

Jules White, its founder and headteacher at Tanbridge House school in West Sussex, said: “At the moment our attention should rightly focus on events affecting the whole country. But even after the welcome government investment set for April 2020, cash-starved schools still require a major long-term funding plan to meet the needs of children and their families over the next 10 years.

“The will from every school and every teacher to deliver a world-class system is there, but the resources are not.”