In my local school district that would be $600,000 a year for a district with 1500 students and 3 school buildings. There are 43 teacher aides on the payroll. Cut a couple from each building and you can pay for a guard.
I call on schools in my line of business. Many have a security desk in the front manned by what looks like retired citizens looking for some extra cash. Their responsibility is to make sure you sign in when visiting the school - great security measure.
Why not man the security desk with a real armed security guard? The only thing lacking is the will of the government run schools to do it.
I think that is a better option than arming teachers who often come into close physical contact with their students. Also if you want a teacher you want a teacher - not a teacher who is trained to engage an armed assailant in a firefight with a room full of children. Something could be said about have a well armed and trained citizenry, but we live in a country of specialists including our law enforcement personal. Even those dedicated to full-time learning on how to respond are vulnerable in these situations (the police officer at the Sikh temple who got shot for example).