Pupils in Bath and North East Somerset will receive less funding than their peers in the South West, new figures show.
The Association of School and College Leaders is concerned many schools are still being forced to reduce staffing and cut provisions to balance their budgets.
Department for Education figures show schools in Bath and North East Somerset will have an average budget of £5,263 per pupil in the 2024-25 financial year – an increase of 10% from £4,793 the previous year.
Budgets vary widely across the regions, with schools in inner London receiving the most per child – £7,065, while those in the South West will only have an average budget of £5,527.
The total budget for schools across England has risen by a tenth since the previous year, reaching £63 billion.
However, the ASCL said when adjusted for inflation, the increase amounts to just 1.9% per pupil.
Julia Harnden, funding specialist at the ASCL, said: "As a result, lots of schools are having to reduce staffing and provision to try to balance their budgets.
"The situation is desperate, and the Government must invest in education in the Autumn Budget."
Ms Harnden highlighted a particular pressure on special educational needs funding, with the total budget allocated to such support across the country falling by 8% this year to £539 million.
In Bath and North East Somerset, £2.8 million is allocated to special education needs support – down from £3.1 million the previous year.
The funding covers services for visual, hearing and physical impairment, specific learning difficulties such as speech, language and communication, as well as severe learning difficulties and autism.
In total, Bath and North East Somerset schools will receive a budget of £153.3 million for this year.
Of this, £6.5 million will go towards funding for high needs places, which provide educational support to children who cannot attend mainstream schools due to exclusion, illness, or other reasons.
Paul Whiteman, general secretary at school leaders' union NAHT, said although the funding arrangements pre-date this Government, Labour must "prioritise schools for investment in the months and years ahead – starting with the forthcoming Budget".
He added: "Schools should be at the front of the queue when it comes to investment, which is vital to the success of the Government's mission to break down barriers to opportunity and ensure children from all backgrounds in every part of the country can thrive."
A Department for Education spokesperson said: "This Government is putting education back at the heart of national life, and is mission-driven to break down the barriers to opportunity for every child, at every stage.
"Departmental expenditure limits for 2025-26 are to be set alongside the Budget on 30 October – and that will include setting the overall level of funding for schools next year.
"Measures such as ending tax breaks on private schools will raise revenue for essential public services, so that every child has access to high-quality education."