The size (or lack of it) worries me. I can visualise having to make decisions about which area of my body to leave unprotected; cover my face and my ability to propogate the human race is threatened, lowering the shield will expose my head to to the threat of a rather fast form of lead poisoning.
Maybe some fast up and down movements could prevent any bullet finding its target. Anyone who is unsure of how to effect this should watch any movie starring Syl Stallone for pointers on technique.
It all seems very American. I imagine that British authorities would issue cricket boxes to all male staff members, and notify female teachers that in the event of an attack they should stand behind a male teacher.
I think it would be cheaper for a school to provide every teacher with a gun. Then they could shoot anyone who walked through the door.