Boy. I agree.There's a gadget I've seen on YouTube which should be in every classroom. Hard to describe, but it's two interlocking plates which slide apart, locking the door securely. There are a number of similar devices. I think the most a teacher should do is to slide something like that in place and then get away from the door with the students...
If we had a country with virtually no firearms, then it might be different, but having a guy like the Florida jackhole on the loose means that while anti-gun folks see schools as "gun free zones," jackholes like Lakeland see "defenseless, target rich environment." I hate that, but that's the lay of the land.
I've had a colleague say that roughly 30% of teachers at Liberty University are packing on any given day (with mandatory gun safety and CCW training). Any school shooting there would likely be brief. They haven't had one yet...