With the perp carrying one and possible two long guns?
That SRO froze.
I’m going to wait to see the (inevitable) video. It’s entirely possible that it might have been a scenario where the officer was at one end of a long corridor, the perp was at the other end, and the SRO just yelled down the hall without being able to see (or maybe not process - people see things all the time but don’t mentally register them properly) details, then when the figure he saw ran, he didn’t think much of it. This happened a lot with SROs and hall monitors back in my high school (which was massive) and I’m told this still happens in modern high schools.