If a student who had a gun on them at school killed an active shooter, would the student be arrested?

It depends! ™

If possession of a firearm on school grounds is a crime in that state, then the fact the student used a firearm on school grounds would be pretty damn good proof of possession. There are DAs who might very well want to prosecute that crime, even in the face of the media and public probably considering the kid a hero.

But just because you can prosecute someone doesn’t mean you always do.