I don't think having an armed police officer at a school is that bad of an idea, but what I don't like is how school administration official's like to use the police to enforce school rules, instead of local laws. Around here, they will have you arrested if you leave school early. And they have the police threaten you will arrest if you are late for school, and just about anything else the school administration doesn't above of.
Several months back, the students were planning on having a walkout, to protest something (can't remember what), well the principal told said that if they did it the state police would arrest them on the charge of "inciting a riot". Well according to rule book, all that needs to be done to hold a protest is alert the principal the day of the event.