I, too, am surprised this hasn't happened already.
Moreover, I'm surprised something hasn't happened in MY school. Ever since I"ve been there, we've had a spread out, unfenced campus. Up until a few years ago, students had to go OUTDOORS to access restrooms located off of breezeways.
Our current school was built in the 50s. At that time, they weren't thinking of safety; they were thinking of allowing the crosswinds to flow through buildings so they wouldn't require A/C. (And now we have A/C.)
Fortunately, we're building a new school, but I know its still a soft target. It will still have only one armed individual in the entire building (a sheriff's deputy). I know of only one other person that will have a CHL. He and I don't talk much about any of this (he & I would not be considered PC on our campus), but I would imagine he will keep his CHL in his truck.
Even if three of us managed to have our PISTOLS in an attack, I don't see them holding up well to a large group of trained terrorists with automatic rifles. To say the least, we would be undergunned--and that's IF the other teacher and I could even make it to our vehicles to get our weapons.
You're all correct--we're soft targets indeed. Unfortunately, we'll have to watch our countrymen get slaughtered, as on 911, to even have the subject considered. Sad.