They should just ask the kids in shop class to come up with something; when I was in school the kids in Shop class were always making Chinese throwing stars from bits of metal studded with nails.
No one ever got hurt, they were pretty safe about throwing them and making sure all was clear; but the darn things were stuck in ceiling and walls all over the school... some even penetrated cinderblock.
Another thread with some 104 comments if anyone cares to scan thru them
http://www.freerepublic.com/focus/news/3646586/posts
Pennsylvania school district arming teachers with baseball bats to combat active shooters