Like most things that get handled by people not competent to run them (No, not the teachers, I mean govt officials creating school policy the teachers have to obey), the issues in the public school system are myriad, and nearly endless in number. (and yes, I know I am repeating myself there.)
Also, like most things handled in this way, problems are dealt with in a "reactive" rather than "proactive" manner, because the people making the decisions dont really know how to run things, and thus do not know how to do the latter.
See also, Dunning-Krueger.
This is why we have had 30 years of reactionary problem solving on this issue, and it has gotten worse and worse every decade. It is not a difficult trend to follow.
Want to fix the problem?
Here's a radical suggestion:
1) Increase the capability of the country to deal with mental health issues
2) Improve poverty conditions in the country with good social services (Which has been shown to greatly reduce incidence in violent crime in aggregate, including abusing home lives, which can contribute greatly to child mental health issues)
3) remove perverse incentives that allow bullying in schools from popular kids. (Ever notice how it is the "most loved" students that get shot first? Ever wonder why?) This means reducing the school's dependency on external sources of funding. (Athletics programs, et al.)
4) Due to implementation of item 1), students can be better cared for when they are identified as being likely to suffer from mental health issues (AGAIN though, teachers are only *AN* authority figure, NOT A HIGH AUTHORITY. They do not get to make these kinds of decisions, they can only alert!!) and can thus be removed from the school and evaluated or cared for by professionals, who do have such authority.
But it is just so much cheaper and easier to tell teachers 'Zero tolerance! Slight infraction == harsh punishment, no exceptions!' and to throw armed guards into the mix to enforce compliance.