1. No
2. If the teacher feels so inclined to carry the responsibility of this, then by all means, allow them to help. Offer to send them to some sort of advanced training ($1,000/teacher).
3. I've always had sort of an internal dilemma when it comes to where the funding comes from for schools. Part of me (the part that doesn't have kids) wants to say, you had the kid, you should find a way to pay for it's education. The other part of me says, kids are the future and should be prepped as such. Short answers: probably tax dollars. Not sure where you should make the cuts, but as someone already said, I do not think it would be too terribly difficult to shift the funds from somewhere else. Hell, I just saw on the news yesterday where the US, once again, sent some ungodly amount of money to another nation for support. Stop this shit, and divert it to education funding.
4. Probably private security. Someone who's trained for just such scenarios. It's probably cheaper to do this than to send them to full blown cop school (or whatever they do). I also feel that one is enough. I had read somewhere that the individuals who perform these mass shootings love the feeling of control, and when confronted by even the slightest bit of opposition, tend to turn the gun on themselves. They aren't there to have a shoot-out with someone. They're looking for easy targets prior to ending their own life. I think moderately trained is sufficient due to this reason. Allow them to dress casual and carry concealed so as to not alarm or disturb the students. After all, we are still aiming for it to be as normal of an environment as possible for the kids. I would, however, keep some sort of heavy-hitter locked up in a specific location that they could have access too.
5. I probably would not allow the students to carry weapons, even if they are 18. They should be there for education, not to defend others. 18 is still pretty young to carry that responsibility.
I'm still not sure how I feel about the subject. I do not feel that it would be too much for a district to set aside $50,000/yr for the purpose of security. Install some serious locking doors and hire an armed security guard to roam the halls for $40,000/hr. To me, that does not seem like that much of a sacrifice to their budgets.