Depends on the size and layout of the campus.
Locked doors that only open from the inside, probably not. Open doors and entrances, probably, depending on the layout of the building.
How many would that be, per school district?
Depends on how many schools, and doors, in the school district.
How much are they to be paid?
I would think the average salary of a teacher, or of a police officer, which are roughly the same, should be plenty.
The same way the salaries of teachers and police officers are currently afforded.
How many "extra" will you need to cover days off, sickness etc?
A very minimal number, possibly none. An elementary school with 3 armed officers can probably get by for one day with 2 in the event of a sick day, and days off are pretty much covered by the weekends, anyway. A larger high school with weekend classes and activities are going to have more officers and work them in shifts, so there would be more than enough to fill in as needed.
How much will the benefits cost?
Too much, same as teachers and police officers currently, because they'll be in a union.
No one seems to want to answer these questions.
Wait. What? Huh?
Don't want teachers and staff armed? Fine, that is OKEE DOKEE. What is your alternative?
Asked and answered. Oh, I see, now I understand why you say, "No one seems to want to answer these questions." It's because they do get answered, but you ignore them. If the answer was, "Let's arm teachers, administrators and janitors," that's an answer you'd hear. Got it.
The problem with this issue is that people want to work it politically, furthering their political agenda (be it pro-gun or anti-gun). Instead, they need to work the problem. In this case, the problem is school security, namely, lack thereof. If you work the problem, it's not hard to conclude that security needs to be increased. How do you do that? You don't do it by simply adding more tools of security, you do it by adding more people trained in security.
Guns don't kill people, people do.
Guns don't provide security, people do.