I suggest extremely harsh punishments for all the bystanders and witnesses to bullying who don't take action to intervene. Punishing bullies doesn't work, punishing victims is unfair, and those in authority aren't capable of stopping it or being present at all times. So punish all the other children for not intervening, or at least tattling, on every occasion of bullying. Bullying succeeds because the rabble doesn't take collective action. If it were in their interests to get involved, the rabble could exercise a restraining influence on bullies. I suggest motivating the rabble by means of punishing inaction. Revocation of desired privileges, suspensions, corporal punishments, expulsions, etc. The initial wave of terror will have to be very harsh in order to make examples of people, to force the herd into line.
What? If kids are going to behave like beasts they might as well be treated like them. If I ran things, they'd each be fitted with electroshock collars. That would stop their nonsense.