So their child was accused of 3D printing a gun and taking it in to school, the parent confirmed online that, yes, their child did 3D print a gun and take it to school ... I'm not sure what you're hoping to achieve.
It's not libel, it's a misunderstanding.
Is it a gun in a meaningful sense if it can't fire a projectile? How could anyone be concerned about that? What if the kid brought toy soldiers to school? Would that be bringing guns to school?
It was a gun in the same way the word GUN on a page is a gun: only symbolically.