While of course everyone is looking at the tragedy of the class(es) that were entered and the students and teachers who were murdered, as I understand it the killing occurred in only those two, joined, rooms. Everyone else survived. If the rest of the school got the word to lock down, why didn’t the two murdered teachers?
Reportedly one or more classes were being shown a Disney film in lieu of “education”. That teachers were among the victims implies that they were in the rooms (as opposed to the old, “start a film and leave for a break” gambit). Two unfounded guesses: 1) The film audio drowned out the twelve minutes of gunfire in front of the school, as well as intercom announcements, if any; 2) the sound from the films attracted the murderer to those very rooms.