Gardaí were called to Scoil Chríost Rí in Portlaoise on Thursday morning after a knife was produced in a row between two students.
Gardaí in Portlaoise confirmed to the Leinster Express that they were called to attend the incident which happened around 10am on Thursday, October 12.
There were no physical injuries and the knife was not recovered after the incident.
It is understood that one student tackled another before the knife was produced.
The school was due to have its school enrolment and information evening for prospective first years on Thursday evening from 6-9pm.
The school principal is yet to comment on the incident.
There are more than 800 pupils in the Presentation Sisters CÉIST Catholic girls school. The number of pupils attending the school has grown rapidly in recent years following its relocation from the town centre.
Scoil Chríost Rí is on the same campus as St Mary's CBS boys secondary school on Portlaoise's Borris Road. The schools are operated on a public-private basis in conjunction with Sodexo.