Where’s the brass?!
There should be no debate about the weapon used in the school, because the floor will be littered by casings. either elongated and tapered 223, or stubby and cylindrical 10mm, or obvious shotshell hulls.
Every first responder will have seen, and every photo will make it obvious.
What makes you think they don't have photographs of it all and didn't pick it all up???
Its standard procedure to photo and collect.
And then what makes you think that they would show off the brass or make the photos public???
They never do that.
The brass will tell the tale, and it can’t be faked, replaced etc, because all sorts of first responders were taking photos without a doubt.