Reward:
Crazy people wanting to shoot up schools will be deterred by the thought of armed teachers
Crazy people who aren't deterred will be gunned down before they can go on their killing spree
Kids have an opportunity to become more familiar with guns as part of their education
Risk:
Teachers will add to the body count during a school shooting rather than subtract from it by hitting the wrong targets
Possibility of accidental shootings
Possibility of crazy person taking gun from teacher to use on their spree
Possibility of not-crazy, just angry, person taking gun from teacher and using it in a crime of passion