I’m guessing we’ve all seen that whimsical stress-reduction kit. It’s basically a sign you hang in a place of your choosing, preferably on a brick wall. The sign says: “Bang head here”.

That, for a long, long time, was what the campaign over the education funding crisis felt like. It was a sign we needed, a sign we used.

But - and it’s a cautious but - I wonder whether the message about the devastating impact of real-term funding cuts may finally be cutting through.

The most recent indication is the fact that education funding has become a central issue in the Conservative leadership campaign. Matt Hancock is the latest candidate to pledge more money for schools, following on from rivals Michael Gove, Boris Johnson and Esther McVey.

Their political radar may be particularly sensitive to this topic because of what happened at the last general election when the issue of school funding caused 750,000 voters to switch support.

We’re into realpolitik here. No prime minister will want to go into the next election refusing to budge on an issue of huge public concern while robotically repeating the mantra that more money is going into schools than ever before. As the Department for Education is fully aware, this is only because there are 600,000 more pupils in the system than there were eight years ago.

So it’s not the total amount of spending that is important but how much funding is allocated for the education of each pupil. And, indisputably, that figure has fallen by 8 per cent in real terms over the past eight years.

Thus the question is no longer whether more money is needed. Most people now appear to agree that this case has been made.

Now it’s a question of how much is enough.

Mr Johnson is reported to have in mind funding of at least £5,000 per pupil for every secondary school in England; Mr Gove has promised £1 billion extra for schools; and Mr Hancock’s pledge amounts to about £3 billion. Meanwhile, Ms McVey has controversially suggested she would cut spending on overseas aid to free up £2 billion for schools and £2 billion for special educational needs and disability and further education.

I suspect many people in education and beyond will feel a deep sense of distaste at the idea of retreating on what many regard as an important international commitment. Or in the implication that one of the world’s biggest economies isn’t prepared to invest fully in its education service without penalising human beings elsewhere.

Analysis shows that Mr Johnson’s figure represents the worst deal, adding up to around £50 million extra, which is a drop in the proverbial ocean compared with the billions that are needed. Mr Gove does a bit better but still falls a long way short, while Mr Hancock is getting warmer but, frankly, is still not there.

Early this year, specialists from the Association of School and College Leaders (ASCL) did some detailed modelling to establish exactly how much money is needed to educate the children’s young people. We called the resulting report “The True Cost of Education” and it concluded that school funding needs to be increased by £5.7 billion.

This was not a figure plucked from the air but a carefully constructed assessment - using Department for Education methodology - of how much it costs to provide a basic expectation on schools for every child in England, irrespective of their background. It’s what most of us believe the state should provide, as a minimum, for children.

So it seems reasonable to us, and what we believe the public would expect, to define the basic expectations on a school as follows:

“It will deliver a core curriculum in a building that is safe and well maintained, put a teacher in front of every class, provide necessary resources and support staff, meet necessary pastoral and safeguarding requirements and provide the first £6,000 of special educational needs support for pupils with additional needs.”

However, even £5.7 billion extra is only one part of the picture. Our report examined the schools budget for pupils aged 5-16. We didn’t look at the high-needs budget, nor did we consider 16-19 funding where there are acute funding pressures.

Thus, the total ask for education will be higher still, and we are currently working on that figure with our partners in other education unions and organisations.

Critics will doubtlessly accuse of us being naive in calling for so much money. They’ll resort to tired “magic money tree” imagery and brand us out of touch with political reality.

We disagree. Our view is simple: that figure is how much is needed to educate a child using a very basic set of expectations. It isn’t up to us to play political games over the education budget, but to provide hard evidence. That is what we are doing on behalf of the nation’s leaders and the young people we are responsible for.

The fact that the shortfall is so vast is not a sign of our political naivety but of the government’s neglect of the education system.

In a letter that ASCL sent to the 11 declared candidates in the Conservative Party leadership race yesterday, we said that schools and colleges require major strategic investment, with funding based on an accurate appraisal of the costs involved in delivering a world-class education system.

The key word is “strategic” because the funding crisis that bedevils our education system has to be resolved by a long-term plan that matches resources to society’s expectations. We cannot keep asking our schools and colleges to do more with less. Our dedicated teachers, support staff and leaders deserve better than more short-termism.

And, more importantly, so do the nation’s children and young people.

Geoff Barton is general secretary of the Association of School and College Leaders