Normal these days for primary and secondary.
Could be for a loose dog, a lone nutter with a machete or Samurai Sword reported in the area, police raid on a house nearby, a gang dispute, a batshit parent, a major accident down the road, terrorist attack, the air ambulance needs to land in the field just before breaktime to reach somebody seriously ill in a house nearby (we've had that one), threats made against the school (like some little shit did to tons of schools last year), somebody's broken their leg on the school field and doesn't need 648 kids making stupid comments about whether they are going to have their leg chopped off - or filming them crying in pain - domestic dispute being taken to somebody's place of work/school, gas leak, large fire, etc, etc.
Generally boils down the partial lockdown rehearsal, where corridors are checked, children are gathered into the classroom and the door is shut - as I understand it, nobody is required to perform a full lockdown/active threat drill (the hiding in cupboards thing) - it's to get the kids able to accept not taking their fucking phones out and calling their Mums to come and get them and not marching out to exercise their unalienable Human Right to have a piss during lesson times. The register gets called, any extras found in the corridor going to/from the toilet are notified to the office, everybody stays in class doing something quietly, calmly and under control.
Difference is that in previous times, kids would listen and just come back inside and settle down quietly, rather than argue and phone their mums. Now we have to make it an official thing in case parents go mad that their darling child was told they had to sit down and be quiet. And because there are so many happening in the US that it's likely somebody deciding to do it here would set off the fire alarm to try and get all the kids out into the open - they have to learn that they have to do what they are told at the time, even if it includes ignoring the fire alarm.
I'm glad we aren't in the US where they have full blown Active Shooter rehearsals for infants with them standing on toilets and piling into cupboards so they can't be seen.