Most bullying relies on the participation -- or at least the passivity -- of an audience, because it rarely serves the needs of a bully to act out where his behavior doesn't get approval and attention from his peers. This means that team-building activities for kids who don't bully can make them a powerful force for preventing bullying in your school.
De-escalation is the practice of taking a charged circumstance and bringing the intensity level down to normal. Junior high kids can work in groups to come up with ideas for de-escalating a bullying encounter, and role-play putting those ideas to work. Older tweens can take this activity a step further by having one participant refuse to calm down, with the other participants finding other ways to resolve the matter.
This takes a page from the sit-in movement of the civil rights era. In a bullying scenario, peers can move as a group to stand between the bully and his target, keeping their backs to the bully, and supporting the other person with words. This requires timing and teamwork, and is much more effective when several kids work in tandem. Practicing standing with one participant each playing the bully and victim is an effective drill.
The Bully Card
The bully card is a game about point of view. Prepare a deck of cards with one that's marked "bully" and one marked "bullied." Deal them out to the whole group. The child who receives the bully card plays the role of the bully, with the victim played by the child who receives the "bullied" card. The group then plays out a bullying scenario through twice -- first with everybody making the worst possible decisions short of actual violence. In the second play-through, everybody makes good decisions. Afterward, the team should discuss both scenarios and how they might apply what they learned.
This activity requires either a prepared set of cards listing various bullying behaviors, or a session with participating students who try to create such a list or card set. Once you have the list, students discuss the severity of different kinds of bullying, rating them from coolest to hottest. Cool in this context means the least harmful, for example a mean comment made in the heat of the moment. Hot would mean the most harmful, such as a systematic Internet attacks or physical violence. The result of the activity isn't as important as the discussion that happens while the students participate.