Hiw many in the 172 out of 1200+ districts that enacted the law?
Answer: 0

intriguing if true. No idea.
No idea, either. But maybe a pertinent thing to figure out before claiming "yeah, but 20 shootings."
Another thing I'd like to know is whether there have been any instances of these marshals leaving a gun unattended in a bathroom or whipping it out because Johnny forgot his homework. These seem to be the leading concerns with arming teachers according to Facebook memes.
The outrage and resistance to making schools more secure almost kinda maybe seems like liberals would rather keep the focus on banning guns....