A first move to provide for the protection of students in public school would be to harden the points of entry. Move the offices and secretarial staff from the often direct path inside the front doors.
Install strong entry doors and bullet proof glass. Create a foyer with a second set of strong entry doors and bullet proof glass. Hire two dedicated and heavily armed guards to handle all entries therein. This would be similar to Prisons and Courthouses, and a few other hardened facilities in the United States.
Once inside the main lobby of the school, all corridors to the classrooms, the classrooms themselves, and the offices should also have strong, locked, entry doors with bullet proof glass, only accessible by an electronic entry system manned by select office staff.
All ground level windows should be of bullet proof glass. If it's good enough for the Presidential motorcade, is it not good enough for our children? What amount of money should we spend to protect them while they are in school anyway?
Strict policies and procedures need to be in place to keep all other access doors to the building locked at all times during the school day. Entry and exit for gym class, and any other reason one can fathom, should be done only through the front lobby.
Of course, the time of student arrival, and departure, then becomes the logistical nightmare.
No amount of armed LE presence can guarantee safety. Not at school, or anywhere else in the United States. However, strong LE presence when the busses roll in, and when they depart at the end of the day would be helpful.
In this incident, if the school had a hardened main door entryway, this killer would most likely never have gained entry. In fact, in most school shootings, the killer enters through the main lobby doors.
I wonder if public school officials would consider the students worth the immediate investment to put these protective measures in to place?