That does not mean I'm qualified to protect children in life and death situations. With a lawyer behind every bullet fired , I hardly think janitors for what they get paid would be willing to take on that roll.
The real twist in this conundrum is the difference between hiring someone to protect people as part of their job and ALLOWING them, in essence, the same right to defense of self and others that they'd have off of school property.
In the case of a police officer, they are trained to follow specific department policies and are indemnified against civil suit if they do follow those policies. The citizens of their town or city (through the city's insurance) take on the risk for that officer's actions while performing his/her duties. An armed guard hired to provide security has a slightly different but similar set of circumstances. He's an employee of a firm set up to handle the fallout of whatever he might do on the job, so long as he's following his lawful orders.
Schools are not set up to provide training for teachers or janitors to use lawful force and they don't have either school-board policies on the use of force nor indemnification of the employee against civil suits resulting from the use of force while on the job.
The real rub, however, is what happens when a school board simply says, "the following employees are hereby permitted to be armed on school property. (period)"? Can the school completely absolve itself from being involved in a civil suit if the employee does something stupid, careless, ... or right and lawful but with some negative consequences? The answer is almost certainly not. You can sue almost anyone you want and the school district will have deeper pockets than anyone else involved.
There is also the question of whether a citizen, who happens to be a teacher or janitor at a public school, would willingly take on the responsibility for discharging a firearm in a classroom -- BUT, I think almost all teachers would rather do so (whatever the consequences might be later) rather than see their students murdered.