Obviously because a teacher's #1 priority is education, their training is primarily in education, the tools/equipment they use are for education, their daily job reinforces education. Whatever gun, ccw or live shooter training/requirements they have would be secondary at best to education.
The basic belief is that more guns = more problems. Cops already have guns because its a necessary tool to perform their job. Even though 18 states allow ccw in schools, it doesn't mean every single school/district allows it in their school. Most say no and many of the one's that allow it have restrictions. (gun has to be in car/parking lot for example) Gun-free-zones also don't apply to cops.
Additional to the problem of more guns is adding to the confusion chaos in a live shooter situation. More cops = more guns, but there's less confusion because we know they have guns and we not only give them the authority to use them but allow them to carry them in gun-free-zones.