The reason zero tolerance policies get so often implemented is because kids tend to be much smarter in how they go about being dicks to one another than in how they do their schoolwork, or, in how they do almost anything else. If you implement one-sided policies that favor the bullied, it's then in the bully's best interest to instead appear bullied, which can end up being a pretty common tactic anyways. There's also more circumstances under which bullying can take place than just physical. Verbal bullying is much harder to prove and do anything about, and the worst is probably when some random kid gets dogpiled for being different, there's not much teachers can do about that even if it's relatively obvious. Which is also resting on the assumption that the teacher isn't also taking part in the bullying as a way to be seen as "cool" by their students, which is unfortunately something that's not uncommon. There's also mutual bullying in which kids can egg each other on until one goes too far, and then maybe zero-tolerance policies end up making some sense, as the group's behavior as a whole is what really needed to stop.

I think taking a more top-down view of the problem, it would seem to me that there's a similar problem going on to when reagan defunded all of the mental institutions, or whatever metaphor you'd wanna use here. There's a lot of attempts to make things right by removing things, rather than adding things. It's bad to lower a student's grade as a result of their malicious behavior, rather than their output, and usually bullies have bad grades anyways. Can't impose on the parents at all because the parents of bullies tend to either be nutso helicopter parents, or tend to be bullies to their children. And then sending kids to other school districts usually just ends up condemning then to a boiling pot of other kids who are maybe worse, or will exacerbate their behavior as it isolates them more, and in extreme cases it can lead them to criminality. Results are going to be kind of mixed on student counseling, if you have a therapist or psychiatrist on campus that's extremely lucky and can also have mixed results, and there's really not anything else you can offer kids other than that, for a variety of reasons. It's relatively hard to get people to stop being self-destructive in the best of times, as an institution, and it's much harder when those people are kids, and when you're inevitably going to be some underfunded institution, since schools funded by the rich, and their property taxes, tend to have children that will engage in less bullying, even if those kids are subject to other psychologically unhealthy pressures.

We could probably solve a good amount of this by just funding schools federally on an equal basis, or with voucher programs based on student population, but nobody wants to fund/expand those programs because schools tend to be underfunded and give bad results already, and we unfortunately have a tendency in this country to give something less money when it performs poorly, as some sort of sacrifice to the free market.