Heads in the south London borough of Sutton have enlisted parents in a bid to fight budget cuts.
Urging parents to email MPs and councillors, the heads told them: "They are not listening to us and we hope instead they might listen to you."
The cuts mean fewer teachers, larger classes, less choice, less support and fewer opportunities, says a statement on local secondary school websites.
The government says school budgets are at their highest level ever.
But, the 16 secondary head teachers say every secondary school in the borough faces cuts to staffing and resources as budgets fail to keep pace with costs.
"Will you help protect your child's education?" the statement asks parents.
It says school budgets have been cut by 8-10% over the last few years "as unfunded pay rises, increased pension and national insurance costs and 'taxes' such as the Apprenticeship Levy have meant schools have less money".
It adds: "This means cuts in other areas of the school to fund these extra costs. The future looks bleak."
One head teacher told the BBC that government plans to resolve funding inconsistencies between local authority areas through a new National Funding Formula for schools is "a diversionary tactic".
And even though Sutton is due to benefit under the new formula "all it will do is partially offset" wider funding problems, said the head, who asked not to be identified.
The move is the latest in a series of campaigns by heads.
West Sussex heads wrote to MPs asking where to make cuts - whether through laying off teachers, reducing school hours or closing counselling services.
Heads in Cheshire warned of a four-day week, the scrapping of some subjects and the loss of support staff.
And North Somerset heads asked Theresa May to reconsider "retrograde" plans for new grammar schools.
The Sutton heads, some of whom run very high performing schools, say they "are close to crisis".
They warn that they face a double budget squeeze from both central and local government, with the local council proposing to claw back 0.5% of basic school funding from September to meet other costs.
"Head teachers are not known for their militancy," the statement continues.
Parents are asked to seek assurances that:
- Schools will no longer be hit with unfunded wage rises
- They will be exempted from the Apprenticeship Levy
Late last year the National Audit Office said state schools in England would have to reduce spending by £3bn by 2019-20.
The BBC has approached both Sutton Council and the Department for Education for responses.
A Sutton Council spokesman said no-one was available for comment.
The Department for Education maintains that school funding is at record levels - £40bn per year in 2016-17, including £2.5bn in pupil premium payments supporting disadvantaged children.
The department also argues that the new funding formula will provide a much fairer basis for allocating funds to schools and will give head teachers more certainty over their future budgets and long-term planning.