(Alex scooped me on the obvious "do a back-of-the-envelope calculation" point; below is the calculation I was going to include in the comment I was drafting)

There are about 7.2 million teachers in the United States. Suppose firearms training takes ten hours per teacher per year, and suppose we value that time at 15 $/hr; that's already more than a billion dollars. How many people die in school shootings in a typical year? Glancing at Wikipedia's list of U.S. school shootings in the decade 2000-2009, I count 67 fatalities, or about 7 per year. But it doesn't seem reasonable to spend $140,000,000 to save a life [...]