It should be a standard protocol that if a parent is calling a school with an extreme emergency that involves a threat of a mass shooting, the police show up at the scene and begin securing it & hardening it from attack like they were called to an actual shooting.
Whether the school should be evacuated at this point is unclear to me because I am not a security effort. Perhaps it would actually create greater ease of targeting in the event of a shooting and be wrong. Perhaps the best move is to lockdown the school and post police at each entry point in addition to forming a perimeter far outside the school... IDK!
... But you would think an administrator would immediately contact the police and without any standard protocol, they would do something like this.
This is a massive failure.