"It was only once the senior sergeant suggested it may have been boys filming, I went to the class and identified that was actually the case."
Mr Mangan said it was unusual for toy replica guns to be on school grounds.
"The teacher had checked what the boys had. One of them had a very colourful yellow, more like what I would see as a water pistol.
"But the other one had a black plastic [gun] that looked fairly similar to what the armed offenders squad had, wandering around our school. He brought that unbeknown to the teacher," he said.
Mr Mangan said the incident had been embarrassing for the school and he had met the students concerned.
The boys would write a letter of apology, he said.
Mr Mangan also noted how well his students and staffed had responded to the situation and he was pleased everybody took the event seriously.
"They [police] advised us to lockdown the school and we did, which means ringing an intermittent bell, with every boy and staff member locked inside.
"When you get a call like that from the police, it is a worry for the health and safety of a large school with a lot of students, who are our responsibility the time they are here. To their credit, the boys responded very well. It was a very valuable opportunity for us to trial our lockdown drill in a realistic situation."
Mr Mangan wanted to express his apologies to police and any members of the public who had been caught up in the half-hour event.
In a statement, police confirmed the school was put into lockdown yesterday afternoon.