Almost all schools in England will have a loss in the funding available per pupil, according to analysis by education unions.
The unions' figures indicate that 98% of schools in England will have a loss in funding for every pupil between 2015-16 and 2019-20.
The National Union of Teachers (NUT) and Association of Teachers and Lecturers (ATL) study suggests that among those hardest hit are schools with children from "just about managing" families - known as "Jams" in Whitehall - the group that Theresa May had vowed to help when she entered Downing Street.
The Department for Education (DfE) disputed the analysis, insisting more money was going into schools and said it did not recognise the claims about Jams families.
The proposed new national funding formula announced by Education Secretary Justine Greening in December increases money targeted at schools with additional needs - including deprivation.
The changes, being introduced from 2018 to 2019, will mean more than 10,000 schools will gain funding.
But the unions' analysis, based on information for 19,719 schools published as part of the national funding formula consultation, indicates 98% of schools will face cuts in per-pupil funding.
The NUT and ATL study also suggested that Jams - which they counted as children who had previously been on free school meals at some point over the last six years - were being hit particularly hard.
The analysis suggests primary schools with the fewest number of Jams faced cuts of £297 per pupil between 2015/16 and 2019/20 while those with the highest number faced cuts of £447 a year.
For secondary schools, the figures ranged from £498 a year for those with the fewest Jams and £658 for schools with the highest number of children from families which were struggling to get by.
The twin pressures of inflation and rising pupil numbers has placed a strain on finances.
The department's schools budget is protected in real terms but does not provide for funding per pupil to increase in line with inflation.
NUT general secretary Kevin Courtney said: "These are shocking figures that will create despair in schools up and down the country.
"Far from being the levelling-up of funding that councils and heads have demanded, the Government is levelling down and schools across the country face real-terms cuts in this Parliament.
"It is impossible to deliver an effective education to pupils if there is no money for staff, buildings, resources, materials, activities or a full subject choice.
"Parents and school governors should unite with teachers in demanding the Government fund our education properly."
His ATL counterpart Mary Bousted said: "All the Government's warm words about protecting the poorest children look meaningless.
"Many schools are already struggling to make ends meet and are desperately trying to raise money from parents for school books and IT.
"These funding cuts will make the situation even more desperate.
"If the Government doesn't increase the overall amount of funding for schools, a generation of children will have a severely restricted education with nothing beyond the basic curriculum and thousands of school staff will lose their jobs."
A Department for Education spokeswoman said: "These figures are fundamentally misleading.
"School funding is at its highest level on record and will be over £40 billion in 2016-17.
"To suggest that we are taking money out of the system is simply incorrect; we are protecting per-pupil funding so where pupil numbers rise, the amount of money schools receive will increase.
"We have also set out proposals to end the historic postcode lottery in school funding. The proposed formula would mean, taking into account the pupil premium, that funding for pupils with additional needs would account for over a fifth of the core schools budget.
"We have also announced a further investment of £190 million to provide more support to underperforming schools and ensure the number of good school places continues to rise."
Liberal Democrat leader Tim Farron said: "Theresa May promised to help the Jams but it is all talk and no action.
"Rather than trying to improve opportunities for everyone, the government is only interested in expanding grammar schools for the few: 98% of schools will have less money to spend on the education of our children, and that is a disgrace."
Shadow education secretary Angela Rayner said: "These are very worrying figures which spell out the impact of Tory policies in our schools.
"The independent National Audit Office have forecast an 8.5% cut in budgets for schools after years of austerity. There is a risk that staffing will bear the brunt of these cuts - and that will have a direct impact on the quality of education provided for every child.
"It all adds up to children being denied the opportunities they need to reach their potential. If you cut school budgets like this you cut the chances that children have to get on in life.
"Instead of obsessing about introducing new grammar schools for a tiny minority of children, the Tories should be concentrating on giving every child the chance to succeed."