This might be a good point to explain something that most folks don't really appreciate:

The resource officer's job is NOT primarily to protect the school and to arrest miscreants. Our current SRO is SWAT-trained and would undoubtedly do his best in a shooting situation, but the SRO's job is mostly preventative. He spends his time working with the discipline vice principal and deans to identify potential troublemakers, establish connections with students, work abuse cases, generate intel on drug dealers trying to work their way into the student body (a much bigger overall threat than shooters, and at every school in the nation every day of the school year), and keep problems from developing. That necessitates the officer being a known and hopefully trusted permanent presence on campus. Ours rotate on a five year tour. In fact, if arrests need to be made, our PD usually brings in other officers in order to not have the SRO's role being perceived primarily as that of an enforcer by the students.

It's impossibe to prove a negative, but who knows how much has NOT happened because of an SRO's work.

Everybody focuses on that deputy's failure to engage the shooter, but his apparent failure to do his job beforehand acting on info he got about the potential of the shooter may be even more egregious. There is no telling whether charging into the building would have helped, but acting on the warning signs might have prevented the whole event.