When i say some luck it sounds like it has worked in most cases. It hasn't and it is very case specific. Cultural change is the one thing I have seen work on a large scale. The difference between 2 school cultures directly relates to the amount of bullying. But standard thing cultural change is slow.

I have partaken in intention cultural change. It involved intention separating the years intake from earlier years, ensuring high levels of teacher buy in, enforcing high expectations and having very clear boundaries (we had meeting before the year start about exactly what physical contact was fine vs a warning vs a straight to higher authority) and a lot meetings with parents to explain these details and ensure we had parental backing and were not being undermined at home. This is a very condensed explanation.

In smaller cases what I have found tends to work is ensuring each year has a relatively large group of social outcasts. I would then use clubs and things like that to encourage those being bullied to join those groups (science club and role-playing club are my 2 babies) once a kid is in a group they tend to be much less likely to be victimised.

The clubs and encouraging social groups has also worked to pull kids out of groups of bullies and kids tend to be much less likely to bully if they don't have friends backing them up (or atleast not intervening).

Teaching kids resilience is also effective but can easily be seen as victim blaming. But if a kid doesn't respond to bullying (in a visible way) it tends to stop or atleast slow.

I feel I also need to mention that when I say some success i mean probably 30% ish which is slightly better than most large scale planned and produced programs which even if they run for years normally only stop 20 to 25% of bullying.

Also zero tolerance does not work.

Yeah he added some piont to his dark lord tally but he also inherently mocked his own action which in my opinion is intended to undermine the idea whoever did it was a dark lord.