Something is making that gun kick hard. It can't be the shooter faking it. It was multiple shots so something was exploding in the barrel to make it kick each time. Maybe a different frame rate on the camera that doesn't catch the ejected brass. IDK, but this is another red herring.
It’s possible that portion of the video was from the active shooter drill performed at the school before the incident actually happened ... in any event it’s interesting how the official body cam video is proven suspect just like the (#Fake) George Floyd body cam footage had the officer putting his gloves on again & again. TBD + WWG1WGA :)
“kicking hard”? That’s what I call perceived recoil as opposed to actual thrust to the rear. All of these popular black guns that don’t have a buttstock in line with the barrel bore want to “rock” when fired. I’ve tested this on the bench in dozens of ways. One of the worst ever was the MAC 10 which tried to roll over backward when fired. That’s where the forward vertical hand grip came from. It’s an effort to hold the front end down. The geometry is stoopid on all of them. The original Armalite geometry was correct but all the later corruptions are just dumb.
FWIW air guns and air soft guns are both made that look and cycle identical to their powder burner versions…