I don't want to make this a long post (so it will be atypical), but one of the articles I was reading this morning really got to me. The Washington Times article was quoting some folks that said the officer that stayed outside Marjory Stoneman Douglas High School wouldn't have been able to stop the shooter because he had only a semi-automatic pistol. Now I don't know what the officer was carrying. He might have had something as nice as a semi-automatic Glock 17 with 17 or 18 rounds loaded in his gun and at least one backup magazine with another 17 rounds. The shooter had an AR-15 semi-automatic rifle with one presumes multiple 30-round magazines.
Now both weapons are semi-automatic and shoot one round with each pull of the trigger. The AR-15 is much more accurate at ranges of 25 yards or more. On the other hand, the AR-15, compared to a semi-automatic handgun, is unwieldy when you have to make significant aiming changes. So both officer and shooter could fire at the same rate.
In a school, you might encounter a long hallway, but one assumes the shooter was not spending most of his time in the hallway. In most areas of a school, the distance advantage of the AR-15 will not be present, and if it is, the person without the AR-15 should be able to back off to a short-range defensible position. Inside a building, one would assume the semi-automatic pistol would in most situations have an advantage or at least be equal to an AR-15.
Then too, you should take into account the relative training of the officer versus a young shooter. Maybe the officer never trained with his weapon, beyond annual qualifications. But he should have had some minimal tactical training. The officer should have had a skill advantage.
Bottom line, the officer should have had a reasonable chance of stopping the shooter. He should not have stayed outside. And folks should not be defending him by saying he couldn't have done anything anyway.