Parents and teachers are to stage a lobby of Parliament today to raise concerns about school funding cuts.
Campaigners said the action will call on the Chancellor, Philip Hammond, to release more money for schools, arguing that they are currently "seriously underfunded".
Ministers have insisted that more money is being pumped into schools, and announced a new funding formula which they say will ensure money is allocated in a fairer way.
Education Secretary Justine Greening has announced an extra £1.3 billion will be found for England's schools from existing budgets, although some unions have suggested this will not be enough to plug funding gaps.
Hundreds of constituencies across England and Wales are expected to be represented at the event, the organisers said.
Six unions, collectively representing the majority of the school workforce, including support workers, are involved in the action.
Geoff Barton, general secretary of the Association of School and College Leaders, said: "We welcome the Education Secretary's commitment to a new formula to address the postcode lottery in school funding.
"But slicing up the cake more evenly cannot disguise the fact that the cake is not big enough in the first place.
"The overall level of education funding is a long way short of what is needed. Schools have already had to make significant cuts to courses, support services and enrichment activities, and there will be further pain to follow without more investment.
"The situation in 16-19 education is even more critical with a level of funding which is woefully inadequate. The Chancellor must provide schools and colleges with the funding that they desperately need to provide the education that young people deserve."
Kevin Courtney, joint general secretary of the National Education Union, said: "This lobby is another indication that the Government cannot ignore the message they received loud and clear in the General Election that our schools are on their knees financially and the public do not accept this should be the case.
"Increasingly, as a joint NEU/Times Educational Supplement survey showed, teachers are now paying for materials out of their own pockets to try and plug the gaps.
"This, however, is a crisis that goes far beyond a quick fix. The Chancellor needs to address this in his Budget by giving schools the money needed to ensure our children and young people get the education in the 21st century they both deserve and need."
School standards minister Nick Gibb said: "The claims being made by the trade union (NEU) about school funding are fundamentally misleading. There are no cuts in funding - every school will see an increase in funding through the formula from 2018.
"The figures the trade union are peddling are based on historical data and do not reflect the situation in our schools today. They also ignore the fact that schools' funding is driven by pupil numbers and, as pupil numbers rise, the amount of money schools receive will also increase."