Sad !
Again with the red flags that were missed or ignored.
SR showed fairly early on that he was disturbed.
Actions could have been taken to get him help -- while he was still a minor, if I'm correct ?
I can't look at the video of SR and his pal, with the slaughtered cats.

We have a rescue cat who initially had aggression issues, but after a few months of patience trusts us completely.
My heart would break if someone were to harm him !
Just
who is this friend of SR's, anyway ?
Hopefully they're not of like mind; but it's concerning that they didn't go to the authorities.
My opinion.
Teachers or school staff who volunteer to conceal carry at school can take the same kind of tests that LE does. They can take firearm training courses like LE does.
Since these school shootings are rare most teachers would spend their entire career teaching without ever having to touch their concealed carry weapon while at school.
I also agreed with you that having an armed LE officer on campus was a good idea.
I still think that keeping these miscreants outside of schools in the first place makes much more sense then having armed school staff. JMO.
It actually isn’t a new proopsal there have been armed teachers in schools in Texas for many years. Some districts are very rural and are not even close to a police department, they train and arm teachers. What could happen now, is an expansion of the School Marshal system, or in Florida they are School Guardians. Sheriff Grady Judd of Polk County has a video explaining how it works.