I went to Catholic School.
We were not allowed to have backpacks, we were required to use the official uniform bookbag.
It was made of canvas w/2 strong handles but couldn't be closed & it wasn't very deep.
The textbooks & notebooks got heavier by the minute lugging them all day long.
We were not allowed to access our lockers in between classes,
even if we walked by them 6 times a day.
There were teachers monitoring the halls and you'd get demerits for opening your locker,
then you'd be late to class which would earn you more demerits, etc.
Virtually no one attempted to access their locker during the day.
I realize many middle and high schools in the USA have thousands of students.
But, in the smaller schools perhaps they could implement something like the school I went to.
It only took one teacher on one end of the hall, one on the other end,
and one at an intersection in the middle.
They were the same teachers that then went in & taught their class,
no extra personnel needed.
Another idea is clear backpacks.
My last comment is, in case anyone missed it, the school we're all talking about
used panic buttons worn by most of the staff, I believe.
That is why the 2 resource officers were able to get to the shooter within minutes.
And, I'll point out that they ran to the gun!