By moving your left arm holding the gun forward and up while grasping the rising barrel with your right hand and moving it from left to center.
-btw If the police are positioned behind the teenager and to his left, turning to his right places his body between the police and the gun and conceals the weapon from their view regardless how he holds it.
You also might consider how 'normal' it is for someone to turn to their right in reaction to sound/stimulis from their left.
And again, absent the orange colored safety tip at the end of the gun barrel, in this scenario it is extremely unlikely the replica gun the teen was holding would be discernible from the real thing.
And how do you get shot in the right buttocks if you are
in any kind of position to fire??
http://everycitizenasoldier.org/id16.html
Answer — you don’t.