I'll preface this by saying I hunted with my uncle as a youth and my experience was a positive one. The game we hunted was given to poor families in Michigan. I was honored to have that experience.
1. Teachers know some students do steal stuff. Some steal food, tests, cell phones, money, computers, tools from the art room, and even passwords for grading programs. They know where the security cameras are and they are savvy. They can secretly video teachers entering pass codes and likely gun locker combinations. Teachers are overwhelmed. We have all, at least once in our career, left things out where we should not have when a classroom disruption occurs. For example, I might not lock up my grading program before stepping between students who are about to erupt into a fight. We do our best, but we are not perfect. It would be STUPID to assume a gun might not make its way into a student's hands through theft.
2. The kind of student who would use a gun in school would have no qualms about grabbing a heavy textbook, chair, bat, etc, and hitting a teacher in the head with it to get at their gun. The only difference is that they need not bring a gun to school, it's already there.
3. When an active shooter is rampaging, officers entering the school know to shoot at someone with a gun. Assuming a teacher is armed, what is to prevent THAT TEACHER from getting shot by the SWAT team entering the school? The job of armed security should be left to an experienced officer who is not afraid to do their duty.
I believe schools should have a system, in coordination with law enforcement, by which a violent student can be added to a no-gun list. That this system should have checks and balances so that an appeal can be made, and the decision reversed/halted for just cause. Time and time again, these violent perpetrators have had a history of brutality in schools that was identifiable, and therefore preventable.
I believe a handgun for defense, in a society that permits guns, may be necessary. I believe hunting rifles are a tool to provide food. In my humble opinion I do not feel an AR15 or any high-powered military weapon is needed for "sport," and unless you are a member of a "A well regulated Militia," as defined in the constitution, it's unnecessary.
Again, these are just the opinions of a teacher with nearly 30 years of classroom experience. I am not a politician, nor do I want to be, but I do have a blog, and a place to share that opinion. If you have dealt with loss in your school, I have a blog post on the topic HERE.