On the subject of school security and preventive measures:
I visit multiple schools, public and private, in the course of being a dad and supervising student teachers. Every school system we've been at, from Jefferson County (where I taught for 14 years) to the most rural systems in Western Maryland have had security systems in place that require you to identify by voice and image to be buzzed in, an entrance that goes into the office after classes start and you have to be buzzed to the other side, or both. Most had SROs (Petal, MS has its own hired officers with cars and uniforms). One township in Ottawa Co, MI requires people to fingerprint check in at a kiosk in the office.
Video released that was supposedly taken next door allegedly shows the shooter just opening the door and walking right into the front of the building.
I hope states can find the money to provide schools without these security systems an opportunity to install modern security measures. Done right, it's not that intrusive unless you're being anxious and trying to bypass it.