I had a think last night dropping off to sleep - a good time to solve mysteries! It is possible, that it was brought into school as a show and tell item - but was stolen as a prank, and that kid hid it under a desk by jamming it into a corner -or even sticking it on with chewing gum. Not wanting to own up it was left there and forgotten about. Years later a slight movement of the desk suddenly releases it onto the floor where it was discovered. Unlikely I agree, but it would be one possible solution to it suddenly falling to the floor.