Teachers have a lot of responsibility already, and on the whole they get paid crap. Advocating for teachers to have guns so they can protect their students is one step away from requiring teachers to protect students, meaning they would have to be trained, certified gun carriers and that's just one more burden on them. Who would pay for their guns/training? There is no surplus money in education in the US.
But really, the answer is in the first post: gun control. As it stands that's the best option. Next best option is to keep things as they are. Worst options are to arm teachers/encourage teachers to arm themselves.