I've gotta tell you that you're very wrong on the it doesn't take a genius. Take a basic school for instance, I'll use a science building on my local university's campus. It's a square, with an outside parking in the center. It's got 4 stories, you'd think it's pretty straight forward, right? Nope. Even in the simplest of buildings, there's hundreds of places to hide, and even still, active shooters don't mean that there is active gunfire, it means there's someone shooting sometimes in the area. People often when they hear gunshots don't know what it is at first, I mean no one ever thinks it'll happen to them, so they might not even realize they are in a shooting situation, and on top of that, everyone seems to be an expert, and you'll get conflicting reports. He went this way may have been valid five minutes ago, or maybe you're scared out of your mind, and don't know what direction he went, you're just too afraid you'll be useless.
In hindsight, sure it doesn't take a genius to realize there was a shooter, but when you put people that don't expect it to happen in real life, in situations where they're hearing 100 different stories, it gets messy fast. Even in training. I ran command in a school shooting training scenario with our emergency services last year, even though there was areas that were clearly in play for this event, directions of the shooter, what was happening, and how it was happening was still confusing, despite everyone involved knowing it was a shooting for sure, and it was somewhere in this directioned area. Imagine when there's no boundaries, a lot more chaos, and no confirmation of what is happening, maybe it's a targeted killing instead of mass shooting, maybe it's just one or two pops to make a statement, and there's no intention for more. You know none of this going in.
Don't underestimate how difficult active emergencies are.
Also, one last note, our standing policy in the event of an active shooter reported, is to contain the situation as best as you can, which is generally a loose perimeter, and we send in a FAST team of the first 3-4 police officers to try to secure the inside while a SWAT team deploys, hopefully trying to find the shooter so tactical teams can deploy more effectively right away. But Hoppah and Mike are right in saying there's an understanding that you secure the perimeter as best as you can, last thing you want is for your incident to expand any further. And from my experience, I have to agree, security, even up to the level of Campus Police are not trained and equipped to be anything of use more than direction or expertise in the layout. Nothing in the way of a hero to save the day.