The man was found trapped under rocks after scaffolding collapsed on a construction site at Fort Street High School in Petersham before 11:30 am Friday.
The worker was repairing the facade of the 170-year-old public school when he was crushed by a sandstone concrete slab weighing several tons.
According to paramedics, the man could not be resuscitated and died at the scene.
Fire and Rescue NSW Adam Dewberry said it will be a “complex operation to recover the person”.
“It’s likely we’ll need to use a crane to lift some of the debris,” Dewberry said.
Dewberry added that it left staff at the venue distraught and students were kept away from the scene.
“The students have been removed from the area and cared for,” he said.
“One of our firefighters took the time to talk to a colleague to make sure everything was okay.”
Parents were shocked by the incident and rushed to pick up their children.
“It’s so very sad, that’s why I went to pick up my son and take it home and comfort him,” one parent told 9News.
A crime scene has been established and NSW Safe Working has been notified.