It happened last night at around 7pm in Co Longford.
A masked man entered the school on Thursday evening armed with what is believed to have been a screwdriver and took a sum of money.
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Nobody was injured in the incident.
A 38-year-old man was arrested a short time later in Longford town and some money was recovered.
The suspect is being detained at Longford Garda Station and can be held for up to 24 hours.
Gardaí are looking to speak with the driver of a van who may have been flagged down by the suspect in the Chapel Street area of Longford as he fled the scene.
Anyone with information is being asked to phone Gardaí in Longford on 043 335 0570.
