I don't think I said shoot the gun out of the hand. If a cop did that, it would be far too reckless, and the guy would have to be a better shot than a sniper.
Fair enough. You suggested he shoot his shoulder, and so knock the gun out of his hand from the force of the .45 impact. A little more plausible - unless the guy reflexively grips the gun.
You also said, "Also, I thought cops wore kevlar under their uniforms.. True - but kevlar does not protect against armor piercing bullets, and few cops have their head protected.
All in all, it was a clean shoot. The only person to blame for this tragic situation was the kid who decided to bring a gun to school and take a hostage. This was a reasonably foreseeable outcome of the kid's own actions. He brought this on himself.