There is no 3 shot rule. It's stupid and will get you kill. You shoot until the threat is dead. Period.SIXVI06-M said:3 shots is most likely just a very well measured response with balancing risk to others to necessity - it's just enough to get the job done - it doesn't say how scrawny the kid is, but of course- you'd think someone who looks like you can snap in half would take less rounds to put down than a walking meat-wall.
That and don't forget, it's in a school, it's a teenager, the scene is surrounded by despairing kids and parents; the police are already well aware that this is could very well be a political and PR shitstorm. Last thing they need is to be labelled as 'trigger-happy' by world news. To be honest, they did good in that regard.
That and consider, there was more than one cop there - 3 bullets to immobilize and neutralize threat. If they saw any further threat- I'm sure the backup can put an extra 9 rounds into the kid and decide that the kid is definitely down.
With that said there are dills that limit the number of rounds in a target. For example: The round dill put five round into the target immediately. Asset if there is other threats.