Unfortunately no one has actually said that. A few examples of what was actually said
I'd be very surprised if it happened.
Sounds unlikely.
I'm pretty sure this isn't what was said.
I doubt the head actually said it.
At least 2 of those are from me, I think. And actually if read my posts I've said numerous times that we need to exercise caution before jumping straight to complain mode when things don't always occur exactly how kids report it. E.g. they give partial accounts, jump on Chinese whispers etc.
So yes, I do doubt it happened exactly as the student has recounted because it seems particularly unlikely based on my experiences of whole school evacuations.
(But thank you so much for turning my point into 'people say you should never believe children'. I'm guessing you'd be one of those parents who call up demanding to report teachers based on hearsay that turns out to be false.)
If there is anything untoward, then obviously it needs dealing with.
And for what it's worth, I'd damn well be telling any of my students off if they were messing on during a whole school evacuation (note. Not a drill! An AVOIDABLE evacuation just after the fire in London, in the middle of exams, causing distress to those about to sit exams, causing huge amounts of administration to explain why exams have been interrupted etc).