Firefighters responded to a small fire at Yass High School on Friday afternoon.
The fire broke out in a tree at the southern end of the school where construction was taking place as part of a redevelopment.
Yass Fire and Rescue NSW and Yass Valley Support Rural Fire Brigade responded within minutes and the fire was extinguished quickly.
Foreman Grant Ashton reported the fire. He was having lunch with a coworker when he noticed flames in a nearby tree.
"My coworker said he heard a cracking sound and then we saw the fire. We managed to put the fire out with our fire extinguisher but then that ran out and the fire started again," Mr Ashton said.
"I used to do a bit of volunteer firefighting so I remembered not to tackle anything over a metre. I felt alright going in.
"With all the recent fires I wanted to report it quickly," he said.
The fire was located about a metre up a tree.
The cause is unknown, Yass Fire and Rescue NSW Deputy Captain Nick Whiting said.
It was unlikely to be a cigarette due to the fact the fire was off the ground in the tree, Deputy Captain Whiting said.
"We don't know what it was. There were a couple of tree stumps there," he said.