This is so simple.
So ...
The school authorities don't know if it's a dangerous device. They can't make the correct call me very time. So, they have to choose between ...
Over-reacting, or
Under-reacting
If they over-react, the WORST that can happen is that one pupil is traumatised by being arrested and having to move schools.
If they under-react, the WORST that can happen is that, next time, it IS a bomb, and children are killed.
Now ... imagine you're a parent at that school.
If I had a child ... and they were at that school ... there is only one possible answer ...
If you even vaguely suspect that there is even the remotest possibility that it might be an explosive device ... you DO NOT pussyfoot around ... you come down on the situation like a ton of bricks. End of!