- Schools are incredibly underfunded. Do you think a whole lot of money will be spent on arming and training teachers? I don't think so either.
- Teachers are already overworked. It would be great if teachers could be paid to just teach but I know several teachers and they seem to provide a full slate of social services in addition to teaching in addition to student discipline issues in addition to, well, anything else their students need (up to and including feeding them, clothing them, and buying them school supplies). Being an armed security guard too goes way beyond what should be expected of teachers.
- There isn't enough training to be had to teach a teacher how to be competent enough with a firearm to pick off one school shooter in the midst of hundreds of panicked kids. If the teacher is a former SEAL that would be a different discussion but ten hours of qualifying range time would mean disaster--even more disaster--than is already unfolding.
- Weapon retention would be an issue. There is enough violence against teachers that having them carry firearms would just add fuel to the fire.
- Providing the service of armed protection is a full time job, a job in which you want your armed protector fully focused on threat assessment, not switching back and forth from teaching physics theorems in between watching and responding to threats.
The bottom line is that if the government wants armed security on school campuses they first need to harden a campus much like they do military bases then hire sufficient, and sufficiently trained, professional armed security personnel (BTW there are plenty of trained former military personnel who would be perfect for the job).