Sad event, but the kid had it coming.
Perhaps it's hard to see from the perspective of a non-gun culture, but from an enforcement standpoint and a gun-safety standpoint, they did the correct thing.
No officer wants to shoot a kid. They have to be trained to shoot to kill. It has to be drilled into their heads, because if it isn't, they hesitate and lives (innocent and police alike) are lost. In the police and military, you are trained to shoot to kill. Not shoot to disable.
There's quite a difference between a police marksman with a scoped rifle lying down behind a hedge dozens of meters away, who's had the time to set up and zero in on his target, who is isolated and alone (or with a single hostage), and a regular officer who's faced with a gun-wielding assailant in a crowded school.
In the former case, having a hostage or clearly attempting "suicide by cop" in an area which the police have secured opens the doors to negotiation and maybe fancy trick shots.
In the latter, the police have to take every measure possible to ensure that no innocent bystanders get killed. Even crack marksmen with pistols will shoot center mass, because the risks of missing when you're aiming at smaller targets like arms or weapons is too great. Shooting the head is optimal, but you go for the body if you're rushed.
This isn't Hollywood. This is real life. No magic life-saving bulletproof equipment (actually... for portable equipment, bulletproof is a misnomer... vests and shields are merely bullet resistant). No A-Team shooting. No funny tricks like blowing up a nearby fire extinguisher with a bullet (with steel cylinders, not even remotely possible), no charismatic angel-faced cops talking down suspects, no fancy improvised lasso work with a police cordon. Just regular schlubs trained to use weapons facing a potential killer.
If cops were better equipped, had tasers that could be fired around corners or taser shells... maybe... but tasers carry their risks. For one, they're not 100% effective. If the suspect is psychotic or hooked up on drugs, he may be able to shake off its effects... or, at the very least, get off a wild shot before going down. And there is still a chance of the suspect dying.