Say school is in session and a threat comes into the building unexpectedly. That threat happens to be a super sneaky ninja or someone who knows how to be swift and silent. An armed staff member is terrified to have to bust a cap in him and flinches easily. the threat quickly jumps out around a corner and grabs the gun and takes it. They now have possession of the weapon and is now going on a rampage. That might not be an ideal situation, but it is always possible.
Armed staff may also send a negative image. Some people would rather go to a place where people can peacefully solve problems. If a place has people with guns everywhere all the Prius driving hippies may think they are crazy and cannot solve a problem peacefully like Jesus and have to resort to deadly force. They may also think if a teacher goes crazy and gets angry, they might go on a rampage and kill everyone they see. However the probability of that happening is low.
To finish off, with all the school violence and shootings, arming school staff is the easiest way to prevent incidents. When there is no police officer in the building to quickly react schools need a line of defense. The teachers made a promise to keep students safe anyway they can. Arming them is easy and