In the instability unleashed by the general election, no uncertainty matters more for schools than their funding.

The campaigning of unions, headteachers, parents and opposition parties pushed the issue up the political agenda, and rueful Conservative candidates reflected afterwards on how often they had to justify school cuts on the doorsteps.

Now, with begging letters to parents, bigger classes and “crumbling classrooms” common media currency, sources close to education secretary Justine Greening have promised a “fresh look” at school funding. So ministers are under huge pressure to find a solution quickly.

To do so, they must tackle two separate but often conflated issues: the size of the overall schools budget and how the money is distributed. On the latter, the Conservatives, Labour and Liberal Democrats all promised a fairer funding system in their manifestos.

The Department for Education consultation on the Tory version - the national funding formula (NFF) - had closed in March, before the election campaign even began.

But this latest attempt to iron out long-standing unfairness and geographical inconsistency in the school funding system - the fourth since 1997 - could again come to nothing.

A ticking clock is the first problem. The NFF is due to come into effect from 2018-19, and heads leaders say that means it must be included in the next budget this autumn. But the government has yet to address the huge controversy created by its proposals with an official response to its consultation. And the snap election left much of the work in limbo.

Malcolm Trobe, deputy general secretary of the Association of School and College Leaders, warns officials “have not got a lot of time to clear the decks” before the autumn budget.

Ivan Ould, chair of the F40 group of poorly funded councils, which have long campaigned for fairer funding in education, is resigned to another long wait. “It will have to be delayed by a year,” he says. “I just don’t see how they will have sufficient time to do this”.

Rebels hold the power

A second problem is the hung Parliament, which has thrown doubt on whether the government has the votes to get any school funding reform through the Commons.

Although the principle of “fairer funding” has cross-party support, the government’s concrete proposals, which would see 10,740 schools gain and 9,128 lose funding, have already come under fire from some Tory backbenchers, worried their local schools would be worse off. The new parliamentary arithmetic means that seven rebellious Tory MPs would be enough to defeat any unpopular plans.

In its current form, the proposed new formula does not even have the support of the F40 group. In March, it warned prime minister Theresa May that she was “in danger of replacing one injustice with another”.

Since then, the Tory manifesto has promised to “make sure that no school has its budget cut as a result of the new formula”. And this week there were suggestions that newly humbled ministers might yet find some more money for schools (see bit.ly/FundOptions).

But could it be too late to save the NFF and a tarnished brand now strongly associated with cuts and backbench discontent? If so, the existing manifest inequity in school funding will remain even longer. But for the critics of the formula, such as the F40 group, going ahead would only lock in new unfairness.

The government could try to reform the NFF, but Trobe warns against any “quick fix”.

“We would be fairly cautious about going for a sticking-plaster solution,” he says. “If you look for a quick fix, you might put some money in the wrong place and once you have put money in, it’s very difficult to remove it.”

Abandoning the funding formula, he says, would mean “you are going to have some schools that are going to effectively collapse financially under this strain”.

“It would be a considerable shame - if not a disaster for some schools - if, at this stage, the government determines not to pursue it, because the existing inequity would just continue,” Trobe concludes.

If there is uncertainty over the future of the NFF, there is absolute clarity over the desire among Conservative MPs to detoxify the issue of school funding. A poll at the weekend found that 750,000 voters who switched parties during the election campaign did so because of school funding.

Tes has learned that education secretary Justine Greening had lobbied for her party’s manifesto to commit to a real-terms per pupil increase in school funding, but was overruled. Instead, the “£4 billion increase” it pledged actually amounted to a significant real-terms cut.

By contrast, Labour did pledge a genuine real-terms increase for schools. For Rachel Wolf, a former Downing Street education policy advisor under David Cameron and an unsuccessful Tory candidate last week, “it is virtually impossible for the Conservatives to ‘win’ on public spending”.

“Labour will always offer to tax and spend more, even if the economy can’t withstand these demands,” she writes. “But it does seem likely they [the Conservatives] will be forced to neutralise the issue of school spending.”

The prime minister reportedly told Tory MPs on Monday that the age of austerity was over and there was no public appetite for more public service cuts. Her proposed alliance with the Democratic Unionists could provide cover for improving school budgets. The DUP manifesto promised to “fight hard at Westminster for a budget settlement that allows for real-terms increases in health and education spending”. If the DUP wins, it would be hard to justify denying more money for schools in England.