Armed guard simply aren't a very realistic answer. The cost alone makes it a nonstarter. Conservatives aren't exactly known for giving teachers big pay raises or raising taxes so I don't think we would see such a funding measure pass from our side. It ends up being all talk.
Even then it is a reactionary measure, not so much a preventative one. We can't simply be reactionary to this problem.
There have been instances where failures of the NCIC system has resulted in people not being flagged when they should be flagged. That's simply unacceptable. It's not always the feds fault either if I understand correctly but failure of local departments to provide and update information in the system.
Schools are notorious for wasting funding, whether it's for facilities, supplies, faculty or sports, they tend to find things that are not essentials to spend "new money" on while ignoring the deficiencies they already deal with.
There are plenty of retired and former police and military with firearms training and skills who'd be willing to work on school campuses. The hours would be regular, and the vacation time during Winter and Summer breaks would be a plus, too. From my experience, schools already have extra security during football and basketball games, so if they can afford that, they can afford full time security.
One security worker per school is also not enough. They need to have enough to cover the campus as well as provide coverage when someone needs to be home or on vacation. If there's an incident, they need backup, so some overlap during morning, lunch and afternoon times when lots of people are coming and going would be needed.
This is one reason faculty -- teachers, janitors, admin staff -- should be permitted to carry. Security can't be everywhere at once, and teachers can be posted at entrances and exits when school opens and lets out. Give teachers who wish to carry the authority to do so.
Pretending the only real solution is too hard or expensive tells me you haven't really thought this through.