Yes, because my DC are secondary age, and I can remember a bomb scare at my school when I was that age. And as far as I can tell, their schools have proper and rehearsed procedures, and would be acting with the police and I'm ok with both those doing the right thing.
Also, if it meant the school was to close for the rest of the day, I would expect them to dismiss pupils to make their own ways home, as that is normal, and because if there was a bomb having more people in the area makes it harder for the emergency services
The instructions for the primary IIRC stated that parents were not to go to the site because of risk of preventing safe evacuation by creating congestion just at the moment it was not wanted. If/when DC did need collection, parents would be informed when and - crucially - where, because depending on nature of incident, some year groups might be taken to local community hall, library, or park.