there is something that doesnt fit really well for me.

when the cops arrive on scene, they cant know there is only 1 shooter. And it's possible that there is 2, every body think about Columbine. It's also possible that there is explosive.
So it's possible there is still another shooter in a classroom with hostages.

As we have see, cops are drilled for this about "clearing" a building like a school.
And with all the doors closed. it's not easy.
Like Lt.Paul Vance say: "checking every door, every crack, every crevice" (saying this, the school nurse Sally Cox, was out just one hour ago and stayed 4 hours in a closet)
[link to www.youtube.com]

Now you need to add to the "clearing" the evacuation of 500 kids.

For me they acted since the beginning that there is only 1 shooter and he was dead. So, how could they know?