Almost every modern school in America has giant glass doors and floor to ceiling windows at ground level. Fire codes require all doors to be capable of being opened from the inside at all times. Few if any cameras on entrances or parking areas. Playgrounds and sporting fields full of kids and completely open access. Interior doors are cheap hollow core junk with windows. I could go on for pages.

All of these things could be designed away and schools turned into prisons where kids would be super safe inside (I could do it in less than a week of work with funds and a good crew)...and the crazies would just hit the school at bus drop off or end of day when hundreds of kids are streaming into and out of the school. Ultimately the only thing certain to end a shooting is opposing force applied quickly and with extreme prejudice. Active shooter training for LE stresses engaging the shooter as quickly as possible since they will turtle up and self execute once they encounter armed resistance 90+% of the time even if not hit.