Just a few things to add. I also disagree with guards in schools. When I debated, there was a theory known as nuke numbing. Now it is a bit of a stretch in that case, but I think it applies very well here.
What happens in "nuke numbing" is the more people talk about something which is shocking or terrifying, to people. The more familar and adjusted they become to the idea. A brief example will simply be to speak to your parents/grandparents about when the threat of a bomb first came out. All the measures they took, etc.. People were terrified to do anything. Now, we have became adjusted to it. Same thing with school shootings. The more the media reports it and it is talked about, the more commonplace it will become. Now, this doesn't just apply to talk.
Think about this, schools may have guards 24/7 and high security b/c of shootings, but they may also have shootings b/c of guards etc. You adjust ppl to the possibility of something, and you aren't just saying it may happen, you are making it possible in their mind.
So I believe by increasing security, you will increase the need for security. As stated above, it will do more harm then good. Violations of rights happen all the time in schools. I also think putting weapons in schools (even if at the hands of a guard), is just asking for more violence and can easily lead to corruption.
Think of your principles when and if you are in school. Now think of a security guard that gets paid by them. Children are hassled by teachers and principals. Now guards will easily know who the bad kids are and harass them also. Personally, if I had ever debated a school shooting, I believe guards present would increase the possibility of me trying something. Especially if guard it armed.
It says violence/weapons are the answer. Either way you look at it.
Making our schools a prison isn't going to solve anything. The environment will only make it worse. We should focus on why the hell kids so young are killing themselves and others. This is a problem with society. A problem with these kids, their upbringing, and mental stability. Not a problem that can be solved with arming administration or guards. By adding guards, you are adding to the problem. You are saying, yes, school shooting will happen and we are ready for them.
Why not school shooting are happening and how can we stop them? If you simple are protecting the school itself, what about at a football game, or the bowling alley, or outside before school. You aren't solving the problem, you are only shifting the focus off the schools.