Eric, You might gain more insight by watching Dr. Gary Slutkin's work on violence interruption:
http://acesconnection.com/video/gary-slutkin-disrupting-violence (20 min video presentation)
His main point is that we learn by observing. He has created a community system that reduces urban homicide rates; it does not include using the police force. It's evidence-based and supported by The White House.
You asked, "Would walking into a school and seeing a firearm trigger a response that would be qualitatively different than that of a student without previous exposure to violence or significant stress? I'm not sure if there is an answer, yet an interesting question."
I can tell you that when I traveled to Europe as a young 20-something and had to deal w/ benign border crossings, the presence of armed guards was very intimidating. I could feel the threat in my body. But another strong component to these experiences was the body language of the border guards. FYI, I did not grow up around guns. However, like many people, I did experience significant ACEs.
I believe an armed guard in a school could send the wrong message and certain students could be triggered especially if they live in a home or community with firearms. However I believe a strong mediating factor is how the officer interacts with the students, as Jim stated. At his school the officer is trained in ACEs and is an "out" ACE survivor himself. This is a potent combination in addition to the ACE/trauma-informed culture Jim has established throughout his school. I believe all of these components have been key to his school's success. As Jim stated, "It doesn't matter what kind of security system you have...nothing is as important for school safety as to build positive relationships with students...."
Having all this said, I will still be interested to hear if Jim's SRO was armed.
Thank you for this interesting topic and discussion.