Has there ever been a school shooting in the US by a staff member or teacher? No, there hasn't been.

Unstable, hormonal, underdeveloped mentally, and undiagnosed kids are very common and dangerous. This is where the vast majority of school shootings come from, or from gangs.

It's horrible that you guys are in a situation where measures like this are required, but it's also not an entirely unreasonable decision given the circumstances. American police don't seem to be arriving in time, gun control would take generations to take effect, this is what is left.

I'm Canadian and my school always had a few armed teachers and guards at our school (I think the ratio of armed staff to students was around 1:100), although that we more due to the the risk of kidnapping of some children belonging to high ransom value families than anything else. I never felt even slightly concerned about the risk of the people misusing their weapons.