It was not a good tactic in this case, but it is standard police operating procedure basically everywhere. The shooting lasted just over 5 minutes. I have never been to an active shooter event where the police did anything that would have made a positive difference in the outcome of an incident of this short of duration. I have never seen so much breathless misinformation about how the police react to situations such as this.
Once the situation got to the point where an idiot with a gun was shooting up a place... the only realistic hope that someone would intervene in less than 5 minutes was if an armed civilian who had the courage, the skill and the opportunity to kill the perpetrator was already onscene.
If we really want to make schools safer we need to make it more likely that armed civilians who are willing to act are at schools. This situation is a typical and example of the folly of depending on the police to provide for the safety of school children or anyone else for that matter.
Would have been nice if EMS could have gotten to the injured.