I don't think we should arm teachers. The majority of teachers are major richardheads with control insecurities. I knew some teachers in high-school that I legitimately would not be surprised if they ended up blasting a kid in the face with a 12 gauge because they were talking too loud, presented the opportunity to do so. I think if a teacher has prior gun knowledge though, they should be able to conceal carry with their own weapon as long as they follow certain protocols and are screened beforehand.
I really don't think schools are actually as big of a target as the media desperately wants us to believe they are. But if they are, then lets lock them down. They should be secure buildings. You shouldn't be able to get in during the school day. And if you have to, you should be security screened by a trained security guard with a gun.