Do you have a link for that? I looked through a bunch of different articles, none of them made any mention of magnetometers. One did mention an armed guard and security cameras.
I read the OP's first link, which mentioned a handgun in a backpack accidentally going off. The school in question of this particular shooting event had a resource officer and magnetometers.
http://www.inquisitr.com/476828/california-school-shooting-update-suspect-had-20-rounds-in-his-pocket/
How is this authoritarianism, and was the armed guard in place before the NRA came out in favor of that?
Yes, the armed guard was in place well before the NRA.
The school district I worked for from 2000 to 2004 had police at each junior high and high school campus. No magnetometers though. I remember one time when there was a mock swat response as a drill to a bombing scenario. During the exercise, the police did such an awesome job securing the facility that we had a $7000 Cisco Catalyst switch stolen while they were there.
Another time they didn't communicate that they were going to be doing a drill during the summer at an elementary school. One of my coworkers was there doing some wiring work in an IDF. He comes out to see some black-clad ninja with an M4 stalking the halls, and the ninja lays him out flat on the ground.
Cops don't belong in schools. Then again, I don't think cops belong much of anywhere.