Nearly 400 principals in Northern Ireland have demanded a meeting with James Brokenshire over the pressures cash-strapped schools are facing.
In an open letter to the Secretary of State, more than half of the school principals here have asked for an urgent meeting to discuss the budget cuts.
The Primary Principals Group said they are "shocked and outraged" that "nothing is being allocated to schools" from the £10m additional funding for education announced last month.
The group, made up of 375 school principals, said that they are growing "increasingly concerned" that their warnings are not being heard and that children are suffering as a result.
The letter states: "The lack of leadership from Stormont, the Department of Education and the Education Authority is resulting in a situation where communication is poor and often contradictory and, consequently, an atmosphere where rumours are rife."
Gary Farrell, principal of Our Lady's and St Mochua's Primary School in Keady, Co Armagh, said: "Students with complex or special needs will be kept on waiting lists for services such as speech and language development, autism services and support staff. We are worried that when James Brokenshire goes to negotiate the budget he won't have a grasp of what we need as he comes from a business background.
"Schools will have a better opportunity to solve these problems if Stormont were to reconvene."
Mr Farrell warned that there would be a devastating effect on his school if the issue of funding is not addressed.
He said: "If things don't change we could well be facing a teacher redundancy. That would mean a class of 20 pupils could go to 30."
The letter renews a call for an immediate increase of at least 7% in school budgets from the announcement made by Mr Brokenshire in April and a guaranteed year-on-year increase in line with inflation for at least the next five years.
The principals also want clarity on how the £50m for education announced after the Conservative-DUP deal will be spent.
Education Authority chief executive Gavin Boyd said: "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.
"Last year, schools overspent their allocation by about £20m."