Train, train, train. It really has to be muscle memory. Do what you have to do to slow that shooter down. Practice over ands over.
If there's some small light in all this awful experience, it's that this SRO will soon be out of a job and never be employed in LE again.
I read on one of the other threads that the SRO was shot (but not killed). So that may be why he failed to engage.