Reaction time is a factor in a case like this. All the situation required was a cop walking up and talking to him, hand casually (heh) on pistol.
"Hey man, let me talk to you. School camera system thinks you have a gun and I gotta check."
Say the kid did having a gun and was looking to use it, no one, let alone a 16-yo, is going to fumble fuck a gun out of his pocket before a cop can draw down on him. I wouldn't bank on clearing a properly holstered and concealed pistol in this case. Maybe with some serious practice? Also, I am not a high school kid.
Most cops carry Glocks, so no safety to hassle with. Don't know what retention system they now favor, but modern holsters allow the user to draw without thinking about releasing the weapon. I see zero risk in this approach.