A group of parents armed with hockey sticks confronted a 14-year-old boy who pulled a knife on a woman close to their children's primary school.
The incident happened at around 8.30am near Dalkey Project School in Glenageary, south Dublin, while parents were dropping their children off.
Panic erupted when the boy brandished a knife in an apparently random incident.
Some parents ran to the school where children were lined up in the yard and urged them to get indoors. It is understood that a confrontation arose between the teenager and a woman on a cycle path close to the school and the boy produced a table knife.
A group of parents confronted him and detained him using their children's hockey sticks until gardai arrived.
No one was injured in the incident, but a number of children were said to be distressed by what they had seen.
One man said his five-year-old son was too frightened to go to school afterwards. Another parent said the incident had been "very upsetting".
Gardai who were called to the school took the teenager to a garda station and his parents were contacted. He was said to have been in tears as he was led away by officers. Gardai are not pursuing a criminal investigation, and the boy is understood to have been visited by a doctor.
Most of the children had already gone into the school building, but it is understood that one class of youngsters had yet to file inside. They were ushered in by worried parents.
Principal Miriam Hurley said in a statement that no child was involved in the incident which had occurred on Thomastown Road outside of school property.
Dalkey Project School was established by a group of parents in Dun Laoghaire and was the country's first multi-denominational when it opened in 1978. It moved to its current location in Glenageary in 1985.