The parents of about half a million pupils in England are being sent a letter on Thursday warning of cuts to schools because of funding shortages.
The heads of almost every school in Essex, West Sussex, East Sussex and Cornwall have written a joint letter warning of budgets at “breaking point”.
They say it will mean staff cuts, bigger class sizes and fewer support services, such as for mental health.
The Department for Education says school spending is at record levels.
The letter, sent to MPs and parents, from the heads of more than a thousand schools, accuses the Department for Education of being “entirely irresponsible” over school finances.
“School leaders from Penzance to Bognor Regis to Eastbourne and onto Colchester are joining together and are united by a common purpose; we all want adequate funding for every school,” says the letter.
It calls on local MPs to support their schools’ campaigns for better funding.
Heads have been protesting that school budgets have failed to keep up with rising costs – highlighting a National Audit Office report of a £3bn funding gap.
The heads were further angered when this week’s Budget allocated £360m extra funds for new free schools and grammars, but nothing more for running costs for existing schools.
The West Sussex school funding campaign said the decision was “little short of disgraceful”.
The joint letter from heads says they are no closer to any “meaningful proposals” to address the “school funding crisis”.
The letter has been sent to MPs across the four authorities, many of whom represent Conservative heartland constituencies.
The letter calls on MPs to vote to ensure that all schools have an adequate level of funding and that spending plans are “credible”.
It warns that schools will have to make cuts in staff, subjects and pastoral care for pupils with mental health problems.
“It is also misleading for the DfE to continue to state that more is being spent on education when in fact, real terms cuts are occurring,” say the head teachers.
The letter to MPs asks them to examine the priorities for school spending.
It highlights £120m for four land purchases for free schools, £384m for subsequently abandoned plans to make all schools academies and £150m for the expansion of grammar schools.
Schools in other parts of England have been warning parents about running out of cash.
In Warrington, pupils from 11 schools have recorded a song about their concerns about funding levels, called “Funds, glorious funds,” which has been put on to YouTube.
The Education Secretary Justine Greening is due to speak to the ASCL head teachers’ union conference on Friday.
The Department for Education says school funding is higher than ever before – more than £40bn for 2016-17 and rising with extra pupil numbers to £42bn next year.
“We are going to end the historic post code lottery in school funding and under the proposed national schools funding formula, more than half of England’s schools will receive a cash boost,” said a Department for Education spokeswoman.
“West Sussex, Cornwall, East Sussex and Essex would all see an increase in funding, totalling over £47 million for schools in those areas.
“We recognise that schools are facing cost pressures, which is why we will continue to provide support to help them use their funding in cost effective ways, including improving the way they buy goods and services, so they get the best possible value.”