>>Shooting a high school student to stop him from shooting other high school students (if you're not the police) legally is murder and must be charged as such. Hopefully, acquitted.
In this US, killing in self-defense when it's clear that it was self-defense to the police is generally, to my knowledge not charged.
Shooting (and killing) another human being who is NOT shooting you cannot be construed as self defense with "self" being the operative word.
For this reason, it is considered murder (justifiable or not.)