The boy was taken into custody after the terrifying incident near the Dalkey Project National School in Glenageary shortly after 8.30am yesterday and received medical attention.
A source told the Herald: "That is the end of the matter - thankfully no one was injured. This is a sensitive case."
Dozens of primary school children witnessed the incident when they were being dropped off at the school yesterday morning.
It is understood the boy -- who was armed with a table knife -- was confronted by a number of parents armed with hockey sticks before gardai rushed to the scene.
"He was running around, not really going for anyone, but not staying still either.
"People didn't know what would happen next," a woman said.
"In the end some parents tackled him and took the knife from him, and he got very emotional. It was very disturbing."
A spokesman for Educate Together, which operates the school, said the incident did not take place on the school grounds but on nearby Glenageary Avenue.
He also stressed the youth has no connection with the school, and that no staff, parents or pupils were harmed.
The children were lined up waiting to be brought inside when the stand-off took place.
Parents with young children attending the school said that their children had been upset by the sight of the youth with the knife.
"One mother who dropped her child in here this morning said her girl was clinging on to her all the way after seeing what happened at the school," said the creche owner.