Most mass shooting happen very quickly. Even if that kid can call 911 instantly, it's not gonna stop a buncha people from getting killed.

The Virginia Tech shooter, armed with 1 x 9mm pistol and 1 x .22LR pistol, killed 30 people (including himself) in roughly 8 minutes. Even with perfect knowledge and alerting there is no way police would be able to get there in time. Similar timelines for many recent shootings, with Uvalde being a notable and egregious outlier -- the cops were on site very quickly but didn't go in, the bloody cowards.

With iPhone or not, the reality is little Timmy is gonna catch two to the chest if something goes down.