Robbers removed what they thought to be lead from the roof of Forthriver Primary School shortly before 11pm on Monday.
It was found close to the building at Cairnmartin Road as pupils arrived for their sports day on Tuesday morning.
It is the latest in a series of lead thefts across Northern Ireland, and robbers often resell the material to scrap dealers for a profit.
However, the flashing at Forthriver Primary is a synthetic lead substitute, which was later discarded.
DUP councillor Naomi Thompson said repairing the damage will cost "a great deal".
"It is disgraceful that this valued local primary school has been targeted by thoughtless thieves who cared nothing for the consequences of their selfish crime.
"This is all very upsetting for everyone associated with the school: the staff, the parents, the Board of Governors and the school children.
She added: "The school principal, Lorna Rankin, said if it had been raining that night or the next day there would have been devastating damage caused to a number of classrooms. By sheer good fortune it has been dry and the school has been saved from that additional harm."
A PSNI spokesman said: "Police received a report two males passing items over a fence at Forthriver primary school.
"Responding officers found a quantity of lead apparently being gathered close by however there was no sign of the males involved."
While attempting to steal the flashing, the thieves also damaged roof tiles and a new shed.
Cllr Thompson, who represents the greater Shankill area, has called for the Belfast Education and Library Board to review security arrangements at the school.