The safety hood is to protect their heads from smaller debris or unnecessary bumps. During an actual earthquake, the students will get under their desks (or nearby table), which would absorb most of the impact should any heavier objects fall near them. If you are out and not near a desk or table, you are advised to hold something semi-sturdy, such as a briefcase or bag, above your head (and not resting on your head), which will also help to absorb the impact of falling objects.
The safety hoods are usually designed to be used as seat cushions when there isn't an emergency. Some of the hoods are also designed to be used as temporary air filters in the event of a fire, in order to block out smoke as the children evacuate.