1) getting rid of "gun free zones," for normal people would keep mass public shooters away...just doing that.
Allowing parents to simply wear their own guns when they enter the building to deliver forgotten lunches or homework, dropping the kids off, picking kids up for medical appointmens......creates a random environment that mass public shooters avoid. They prefer unarmed targets, they are not looking for a shootout, and won't know who or how many people will be able to fight back against them.
2) Arming some of the staff would also help.
You don't have to arm all the teachers or even most of the teachers...you simply have to let the community know that some staff will be armed, but their identities will be known only to the school administration. This will again scare off mass public shooters. due to the fact they won't know who or how many people will be able to fight back if an attack happens.
3) Have locked exterior doors and two door entry systems for the building. Visitors only enter thru one door, into a lobby, which they are buzzed into, and then they have to be buzzed into the office to get into the main building area.
4) Put in police call boxes alongside fire department pull stations. Local schools in my area have this. The boxes, when the lever is pulled, go straight to the local police station, bypassing 911......and from their every cop in town heads to the school....saves time and the police know to enter the building right away.
These are the things you can do to make schools safer.