There are 1.5 million concealed carry permits in Florida alone. All of these require training and certification. Many of these people practice regularly at the range and compete in action shooting. They shoot way more than ordinary cops. The OPP have to shoot 200 rounds a year to stay current. Montreal Police is a mere 50 a year , and a lot don't even show up. When I was a competitive pistol shooter, I would shoot over 200 rounds on a week-end, never mind a year.
In the US, it is illegal to reveal your concealed weapon unless you have a purpose to draw it: You can actually go to jail in that case. So no, a student is not going to grab the gun off a holster or out of a desk (you have absolutely no idea) because the holster is inside the trouser belt, or on a holster on the ankle, and must be out of sight at all times.
The real purpose of having 20% of school staff with concealed weapons is deterrence. The place ceases to be a gun-free zone and a magnet for crazies. They generally go for easy targets, and not those that shoot back.
Go back to the terrorist attack at a Nairobi shopping centre a few years ago. A lot of people were saved by local civilians licensed for handguns. They were erroneously portrayed as plain-clothes police officers by the ignorant press. When the shooting started and hostages were taken, they went back to protect shoppers from the terrorists. Turns out that the local police, the ones with the AK-47's, were very slow responding and displayed a bit of incompetence when they actually got there.
"But after arriving at the mall Haji was quickly thrown into the heart of a rescue operation – led by a group of civilians with access to weapons and several plainclothes police – that saved
hundreds of shoppers long before the security forces proper took over.
"Suddenly everybody was armed," Haji, himself a licensed firearms carrier, recalls. "Instinctively we decided to give cover to the Red Cross guys."
https://www.theguardian.com/world/2013/dec/08/abdul-haji-westgate-mall-nairobi-kenya-2013-interview
