I suppose it depends on how it's done. I just don't want elementary schools to start looking like prisons the way high schools do.
It also raises some logistical issues. How many elementary schools are there in America? How often does this happen? Then calculate how many police officers are going to spend days, months, years, etc essentially sitting around waiting for nothing.
Each of our schools at the secondary level have 1 - 2 from the Sheriff's department on duty during school hours, our elementary school's share officers if they are within a mile of each other.
The secondary schools have an office at the front of the schools near the main office and they have video cameras on each of the doors, and hallways. They are usually in the office watching from there, and then if we have word that there may be some trouble with fights they will make rounds.
At the elementary schools they are patrolling between the schools, and the Principal, and Principal's secretary and AP's have the tv screens in their offices.
All outside doors are locked except the front main doors, but you have to be buzzed in as far as the next set of doors. (IMO, the glass needs to be replaced at all schools, and that is something that the Federal Government could help the states with (that would actually be good pork)

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We don't have metal detectors, but some of the HS in our area do....
I really don't see the security all that much....out of sight, out of mind most of the time.
I see them more at our sporting events, where they are all working.