Would you feel safe putting a sign in front of your home saying, "This home is a gun-free zone"? Law-abiding citizens might be pleased by such a sign, but to criminals it would be an invitation.
If we had a free-market in education and the Second Amendment was respected, those parents who did not trust teachers with guns would send their children to gun-free schools and those who felt that a school with armed teachers provided a safer environment would reward schools who provided that service with their dollars. The training and evaluation of teachers could be done by private sector institutions of course.
Our public educational system is one of coercion — people are forced by the government through taxation to support schools. So, I could understand the objection of anti-gun parents to legislation which arms the teachers and administrators of their child's school. Again, this is yet another example of how a private, voluntary system would be more effective and responsive to parents' wishes.