Just my opinion, you are entitled to yours, I am entitled to mine. If you are going to place the responsibility to protect students via firearms on a teachers shoulders then they should have the same training as an armed guard at a minimum. I can argue the 80%+ of the people that currently carry are incapable of actually using the gun to protect themselves, let alone others, and do so without causing collateral damage.
How do you propose the weapons are secured amongst the students? I'm assuming you have been in a public middle and high school lately? They can't manage the behavior of the students very well(due to the parents), I don't see where having guns available to be absconded with is a good thing.
"my opinion"
You're right, you are entitled to your opinion and I agree that the teachers who would want to carry should receive proper training. I would like for everyone that carries to be properly trained on how to use their firearm. However, I do not feel I have the right to tell someone that they are not allowed to defend themselves because they haven't been trained. That is a God given right, and no one should be able to take it away.