Troops stationed outside schools sends a message all right. I don't think it's the right message at all.
Here's a few thoughts on why I think this is a bad idea. One, the image of military standing outside schools is just horrible. And on most of these types of security missions, not all troops are issued ammo because of the security risk it entails.
Two, since this would be under T32 status to avoid posse comitatus laws, soldiers would be on state active duty pay, which varies by state. For most, it's much less than active duty pay & doesn't come with benefits. That matters to soldiers who have to leave work.
Three, the National Guard is raising readiness requirements across the board. This involves more training days. Tack on missions like this and you'll put more strain on an already strained part time force.
Just my two cents. I fail to see how this would do anything other than cause more problems while not doing much of anything to solve the problem of mass shootings.
Oh, & this would essentially bring about voluntary mobilizations of NG personnel in practically every unit in the country. Would destroy unit readiness completely. Commanders could kiss their training plans goodbye. Not good when you consider NG is nation's operational reserve.
Also: 
