Almost half of the school principals in Northern Ireland are seeking an urgent meeting with the secretary of state over budget cuts.
The Primary Principals Group say they are "outraged" that of £10m in extra funding announced last month, "nothing is being allocated to schools".
The group is comprised of 375 school principals, who are calling for an increase in school budgets.
The Education Authority said schools are under financial pressure.
"We're quite clear that schools are telling us through their plans that they will overspend their budgets - taken as a whole they will overspend their budgets by something to the order of £20m this year," the authority's Gavin Boyd said.
"So that's the situation we find ourselves in. Last year, schools overspent their in-year allocation by just about £20m."
The BBC has contacted the Department of Education for a response.
The group of principals say they are "increasingly concerned" that their warnings are not being heeded and children are suffering.
They add that "a lack of leadership from Stormont, the Department of Education (DE) and the Education Authority is resulting in a situation where communication is poor and often contradictory".
They say they have written to various officials and politicians on two previous occasions this year regarding the "concerning cuts being made to school budgets" and that during the six months since their first letter "there has been no improvement in the situation".
The group have renewed their call for an immediate increase of at least 7% in school budgets from the announcement made by Northern Ireland Secretary James Brokenshire on 24 April and a "guaranteed year-on-year increase to school budgets in line with inflation for at least the next five years".
They are also seeking clarity on a number of issues including accessing budget surpluses from the last financial year, a "new layer of management at the Education Authority" and how an additional £50m for education announced after the DUP-Tory deal will be used.
A similar letter has been sent to the Permanent Secretary of the Department of Education (DE).
The Education Authority's Mr Boyd denied that a new level of management had been created.
"We've taken five education and library boards and brought them into a single education authority," he said.
"In the course of that transition we have actually reduced senior management levels by 50%."
He said the additional £50m for education was speculation at this point.
Gary Farrell, principal of Our Lady's and St Mochua's Primary School in Keady, County Armagh, said the group felt compelled to send the letter as they were at the "chalkface" and dealing with the difficulties caused by budget cuts.
"In the absence of a Stormont government, we have no representation," he said. "We have to rally together as a professional body to make the general public aware there is not enough money in the system to make it work effectively for our young people.
"How can schools be expected to produce an effective education system if the funding and resourcing is not put in place that they need?"
He also warned of the potential impact for his school if the issue of funding is not addressed.
"If things don't change, we could well be facing a teacher redundancy," he added.
"That would mean a class of 20 pupils could go to 30."
Pamela McKenna, the principal of Knockbreda Primary School, said schools were "at the point where the situation is becoming untenable".
"Senior managers are on their knees with the workload we are experiencing," she added
"We cannot continue to make savings, we cannot continue to cut, these children are the future of our country and without putting significant support into schools we are going to lose the excellent education system we have had for years.
"I don't think the schools are letting the children down, it's coming from the people with the money."
In August, it was revealed that money from the DE which enabled schools to offer more subjects to pupils is to be almost halved.
Schools who work together to deliver the "entitlement framework" were told that their funding to deliver it is to be reduced by 43.5%.
The cut is a cash reduction of £2m from the 2016/17 budget of £4.9m.
A letter from the DE to principals said the cut is "due to pressures on the education budget".
In September, the DE revealed that it is facing £105m of cash pressures in 2017/18.