School budgets in Northern Ireland are so squeezed that some principals are struggling to pay for heating, a teaching union has said.
Paul McClenaghan of the National Association of Head Teachers (NAHT) said schools were faced with the worst budget crisis in 40 years.
He was speaking following a meeting of head teachers in Londonderry.
In September, the Department of Education in NI said it was facing £105m of cash pressures in 2017/18.
Mr McClenaghan, the NAHT president and the principal of Northern Ireland's largest primary school, Strandtown in Belfast, warned of the scale of underfunding in schools.
He said staff have been forgoing salary increases and principals were taking on caretaker roles to reinvest the money into school coffers.
School pastoral services such as counselling and even heating and lighting in schools have also been impacted, Mr McClenaghan added.
He said balancing the books is "a constant struggle".
"My own school is an old school, a hard building to heat. We have to make sure that we garner every penny to spend on our heating.
"The heating system doesn't work very effectively, we can't afford to replace it."
He said schools were regularly asking parents to plug the funding shortfall and said the political stalemate at Stormont was compounding the crisis.
"It has been the experience in my own school for a long time where classroom assistants have bought uniforms, teachers have bought pencils and pieces of paper," he said.
"The Parents-Teacher Association have to make up the gap between the funding because we cannot give children the books they need, the material they need to do their education."
'Mitigate the impact'
He said parents were now paying on average £1,200 per child to make up for the shortfall and that school funding in Northern Ireland fell below that of any other part of the UK.
"More money needs to go into education and that money needs to go directly to schools. Every child needs an amount of money that sustains their education," he added.
In a statement, the Department of Education said it was acutely aware of the financial strain on schools and other education services and has "been working closely with colleagues in the Department of Finance in seeking to mitigate the impact of these pressures in this financial year."