I am going to the question of where all the students were.

From the addition of this video it seems that for the most part the school remained in lockdown. The teachers were told to stay in place. The only physical evacuation, outside the building, was only for those rooms where shooting occurred. Only ones evacuated for a time were those alive in the hit rooms. The term ‘evacuated’ then covered those students alive, both the few taken to the firehouse and the many in other classrooms.

I am not totally sure. File this as good ‘best guess’.