The Army has defused a pipe bomb at a Catholic school in north Belfast.
Dominican College has been targeted twice in a week
The police said the device, left at the gates of the Dominican College at Fortwilliam on Thursday, was made up of firework material.
The area was cordoned off as an Army bomb disposal team moved in to examine the device. It was later made safe.
Last week, the school was disrupted when a hoax device was left at the gates.
Dominican College principal Sister Rosaire said it was "becoming a disturbing trend".
She said she was saddened that people were intent on disrupting education.
"We are appalled at that really, that young people would be used in this way and fear instilled in them," she said.
"Our whole emphasis is in trying to build up a healthy and secure environment for our children and we'd hope that would be there for all pupils in all schools."
Security Minister again been the focus for a security alert.
"Leaving a pipe bomb, or indeed any device at a school, is totally unacceptable and is an attempt to deny children their basic right to education," she said.
The MP for the area, the DUP's Nigel Dodds, said attacks on schools must stop.
"It's another appalling example of an attack on a school which should be a place of sanctuary for pupils, regardless of their denomination or their religion," he said.
"We've had, in north Belfast, the Boys' Model last week having to be evacuated now the Dominican College and where-ever it's emanating from and whatever excuse is given, there can be absolutely no justification.
"The people behind it should desist because they've absolutely no support in the community whatsoever."
The attack is one of a recent series against schools in Northern Ireland.
Sister Rosaire: "We are appalled"
On Tuesday, the Army dealt with a security alert at a school in County Antrim.
A suspicious device was found taped to the front door of Larne High School. A controlled explosion was carried out.
More than 1,000 pupils at the school, as well as Moyle Primary and Nursery schools which share its grounds, were told to stay at home because of the security alert.
The suspicious object was later declared a hoax and police said paramilitary involvement was not suspected.
Also on Tuesday, an Irish language school on the outskirts of west Belfast was damaged in a fire, thought to have been started deliberately.
The alarm was raised at Naiscoil Thaoilinn in Poleglass.
One temporary classroom was destroyed while another was damaged.
It was the second time within a month that the school had been attacked.
The previous attack caused damage put at at more than £5,000.