One person who’s supposed to see to the safety of a huge complex of 3000+ students. It doesn’t matter where he/she was as it is impossible to be everywhere on that campus.
You are assuming they were on campus...
While being everywhere is clearly impossible, being somewhere on campus and responding to the shooting is preferable to being nowhere to be found.
Who was the SRO?
Where was he/she when the shooting started?
What were his/her actions once the shooting started?