My wife has handled a number that no one would dream of giving access to a knife or anything similar. It's a spectrum disorder that can have a wide range of degrees and some very strange characteristics especially in relationship extreme behaviour as all sorts of rather strange things can cause incidents of this. They'd appear strange to us anyway.I know many people with Autism not one would do this, that is not an excuse.
Me I wonder if an extremer case can usefully use a normal school without some one noticing or even if they should be there.
I suspect a clue on why can be found by looking at studies of US school shooting and why the person did it. That appears to relate to bullying and social interaction problems maybe even romance. I didn't look hard just noted one in Britannica
School shooting | Definition, Examples, Causal Factors, & Facts
School shooting, in the typical case, an event in which a student at a school shoots and injures or kills at least one other student or faculty member on the grounds of that school. Although school shootings occur worldwide, the United States has been the scene of the vast majority of the attacks.
www.britannica.com