Schools in Northern Ireland are struggling to provide basic equipment such as first aid kits, stationery and hand soap, MLAs have been told.
The comments came as representatives from the NI Teaching Council appeared at Stormont's education committee.
Dr Graham Gault, from the National Association of Head Teachers, said it will "take a generation" to solve problems facing the sector.
The Department for Education's budget has been cut by about £70m, or 2.5%.
The new education minister, DUP MLA Paul Givan, has met unions in recent days about a range of issues facing the sector.
But Dr Gault said it was of the "highest urgency" that funding for schools is given priority in the budget the executive will set for 2024-25.
"It cannot be the case that what was expected of schools 13 years ago is expected now - they don't have the resource," he said.
Dr Gault said also some schools are unable to replace out-of-date ICT equipment.
"This is basic stuff, even keeping the lights on, we have to do so much better," he said.
