Treating bullying as everyone's problem reduces incidence in primary schools

I always found it really frustrating that a "zero tolerance" policy to bullying seemed to disproportionately affect people who eventually fight back.

I would guess it's a combination of "nobody sees the first hit" (since your attention is elsewhere, of course) and that bullies get quite good at testing boundaries and thus know how to avoid detection.

But, really, it's truly frustrating that as I child I was bullied relentlessly, and when I finally took my parents advice and stood my ground, I was expelled from school (due to zero tolerance). Those bullies continued to torment some other kids, of course.

This is far from an uncommon situation, over the years I've heard many more scenarios like this.

Defending yourself is always the right move. When one of my sons was being bullied in elementary school, I taught him how to fight and encouraged him to do so. The bullying ended, but he was suspended. I confronted his principal and got her to admit that she would defend herself if someone was pummeling her. She didn't like this, and subsequently expelled my son, who later won honors awards after transferring to a different school.

I absolutely agree with you but I can't avoid noticing an extremely common, yet pervasive irony.

The rules of the school no doubt forbid physical violence and expect children to use a process set up by the system to defend themselves against bullying / being wronged. That system failed and because you rightfully saw your son's right to self defense more important than following the rules, you encouraged him to defend himself outsides the confines of the system and its rules.

Later both you and your son were bullied / wronged by the principal. The rules of the state you live in ("laws") no doubt forbid physical violence / being wronged. That system failed...