A school in Dublin was shut on Tuesday after alleged online threats were made against pupils and staff on social media.
Management from Ardgillan Community College sent a message to parents informing them that the school would be closed as Irish police investigated material posted on Instagram stories.
It is understood a student allegedly made references to bringing a gun to school.
Gardai confirmed they are investigating and a 16-year-old has been arrested.
A spokesman said: "It's understood the material was circulated online yesterday evening.
"Following the reports, an investigation was commenced and a 16-year-old youth has since been arrested.
"He is currently detained under Section 4 of the Criminal Justice Act, 1984 at a Dublin Garda station. The investigation is ongoing.
"Gardai are liaising with management at the school and are now satisfied that there is no imminent danger to staff or students attending the school."
Dublin & Dun Laoghaire Education and Training Board confirmed that a school in north County Dublin was closed on a precautionary basis arising from a matter which came to the attention of the school management and which has been referred to the Gardai.
A spokesman said: "There are no further issues arising from this matter and we anticipate that the school will reopen as normal tomorrow."