For example, "zero tolerance" is as much about ensuring so-called "equal treatment" as it is "safety". For fear of being accused (and sued for) bias in disciplining students, schools fall back on lock-step policy rather than reason.
In my state (MI), zero tolerence is 100% the fault of the legislature that drafted various school weapons laws and the governor who signed them (For those that lived in MI, it was Engler (R) and, IIRC, the legislature was controlled by the Republicans). I am sure that some schools would have implemented their own zero-tolerence, but many would have preferred to use common sense and their own discretion.