Hundreds of head teachers from England and Wales are due to attend a rally in central London later, to demand extra funding for schools.
They will meet in Parliament Square before delivering a letter to No 11 Downing Street, amid concerns over work conditions and overcrowded classrooms.
The heads quote the Institute of Fiscal Studies' claim that per pupil funding has fallen 8% in real terms since 2010.
The government says school funds will rise to a record £43.5bn by 2020.
And ministers argue a new funding formula will bring more cash to schools.
But heads are warning of:
- bigger class sizes
- staffing cuts
- reduced subject choices
- loss of support for special needs and pastoral services
Teaching union members, parents and staff have taken part in numerous protests about the budget squeeze over the past few years.
This rally is different as it is entirely made up of the people usually running schools and helping to set their budgets.
Organiser and West Sussex head teacher Jules White said those coming on the rally were "joined by a common desire - and in many cases desperation" to see their schools fairly and adequately funded.
'Cut to the bone'
The heads say the government's approach to school funding is "simply not good enough".
They wrote to parents across the country earlier this week to explain their absence in school.
Geoff Barton, leader of the ASCL head teachers' union, will be attending the protest.
He said the planned gathering was unprecedented and "demonstrates the strength of feeling".
This is the latest push in a campaign by school leaders to highlight the dangers of budget shortfalls and the different funding levels between schools.
Education Secretary Damian Hinds has recognised that funding is one of the biggest concerns in his brief.
A Department for Education official said: "There is more money going into schools than ever before, rising to a record £43.5bn by 2020 - 50% more in real terms per pupil than in 2000.
"The OECD has recently confirmed that the UK is the third highest spender on education in the world, spending more per pupil than countries including Germany, Australia and Japan.
"Every school attracts more funding per pupil through the National Funding Formula, high needs funding has risen to over £6bn this year, and the 3.5% pay rise we announced for classroom teachers on the main pay range is backed by £508m government funding.
"We know that we are asking schools to do more, which is why we are helping them to reduce the £10bn spent each year on non-staffing costs, providing government-backed deals for things like printers and energy suppliers that are helping to save millions of pounds."
But shadow education secretary Angela Rayner said: "This unprecedented action by head teachers is a clear sign of the desperate struggle they now face to provide a decent education while balancing the books."
The head teachers taking part in the protest are from areas including:
- Blackpool • Bolton • Brighton and Hove • Buckinghamshire • Cambridgeshire • Cheshire • Cornwall • Cumbria • Derbyshire • Devon • Dorset • Dudley, West Midlands • East Sussex • Essex • Gloucestershire • Greenwich • Hampshire • Hertfordshire • Kent • Kingston-upon-Thames • Lancashire • Liverpool • London • Norfolk • Northampton • Oxfordshire • Peterborough • Poole • Portsmouth • Richmond • Rotherham • Sheffield • Slough • Solihull, West Midlands • Somerset • Southend • Southampton • Staffordshire • Suffolk • Surrey • Thurrock • Walsall • Warrington • West Berkshire • West Sussex • Wigan • Wiltshire • Wokingham, Berkshire • Wolverhampton • Worcestershire
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