15 years old in 8th grade is not all that wacky. It means he will graduate high school at age 19, a year older than most, and 2 years older than a very few. Perhaps he was held back a grade, who knows, but that age is not that crazy to think about.
As for scenarios that let the kid live, it's pretty hard to judge beyond the facts that we see in the article.
Fact: The gun the kid had was convincingly REAL looking.
Fact: The kid had what was easily confused for a REAL gun in a SCHOOL - and this has been an area of particular concern over the past 20 years due to various tragedies relating to school violence.
Fact: The police yelled for the kid to drop the gun before shooting (this is corroborated by teachers).
I suppose there will be more information coming to light about this though I suspect that there is precious little else to say that would change the salient facts of the story or else it would have been splattered all over the media by now. We shall see though.
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If ignorance were painful, half the posters here would be on morphine drips.
Everyone playing WoW knows everything about playing two classes: 1) their own and 2) Hunters