"That’s old school. I don’t know of anyone using the wait for the cavalry anymore. Whoever gets there first goes in." I'm not so sure that's completely accurate. When the Hmong went nuts in upstate NY earlier this year (maybe it was late last year), the police waited quite sometime before making entry into the building, where of course they found the man dead by his own hand.
And then today, it hasn't been totally clear, but I believe that the first responders waited for some time before making their way up to the floors in question.
Police departments should reevaluate their policy of waiting until reinforcements and SWAT arrive before attempting to make entry. That is valuable time and excess opportunity for the killer to kill more people, IMHO.
I can only speak for the training conducted by my PD, but our active shooter training mandates that there is no waiting even if you are the only officer on scene. Our protocol comes from new techniques that come from some of the mass school shootings we've seen in the recent past. I know of very few PD’s that don't train the same way. That is not to say that there aren't some stuck in the old ways of doing things. There is a much higher chance of an officer being injured now under the active shooter response, but that is part of the job.