It may be another bell ringing in a primary school but the teachers and students at Whanganui's Mosston School know how to react when that bell sounds on and off for a minute.

It's that intermittent ringing which sounds the alarm and it's something the school family practise with every term.

School principal Michelle Watson said the drills were a vital part of the school year and were used in case the school had unwelcome visitors, there was a fire or an earthquake.

"We never know when any of those situations will arise," Ms Watson said.

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And it's during those times that the school goes into lockdown.

"Teachers always have their cellphones with them and active alerts of a lockdown come through on their phones," she said,

All the pupils know that when that alarm sounds they have to stay where are, whether that's inside or outside.

"But then the staff will gather them up into a break-out room and we'll contact the police," Ms Watson said.

Once in the sanctuary of the breakout room the staff do a head count and make sure all the pupils are accounted for. Meantime the Whanganui Police will be called and will take charge.

At the most recent drill, Senior Constable Paul Miller was on hand to make sure things were running as they should.

He checked the school thoroughly which meant going through all the rooms and ensuring all doors were secure and children couldn't be seen.

One of those checks saw him knocking on a classroom door.

"If the children are in there, they know they're not to open the door to someone knocking. They know how to respond and when to respond. This is what the drills teach them," he said.

Senior Constable Miller said every school was obliged to have a lockdown policy and procedure in place.

So how did the staff a pupils at Mosston School score in their latest exercise?

"They're very well drilled," he said.