All these suggestions are wonderful and will be helpful as you grow your story and your characters. But like Debbie says, reactions, etc. depend on the kid.
When we write for kids it's tempting to write a story to get a message about something like bullying across. But stories we love tend to be stories first, about a child on a quest for something, and we follow that kid because we're rooting for him/her to "fill in the blank." By keeping the idea of a theme (could be bullying) in the back of our minds as we write, and NOT out and out writing about IT, the issue will more than likely come up organically and show what we want to show without feeling constructed.
Usually that's a more successful approach than beating the reader over the head (ha) with what we want to say. A character whose ONE AND ONLY trait and role in the story is being a bully is a missed opportunity. Rather than thinking about the bullying scene and the outcome first, maybe consider what brought each character there to your story, and then to that moment. The bully (antagonist), the protagonist, and the observers. Maybe approach each as a person. Who will play the role of antagonist? Why? Because of this, this, and this. What did he/she wake up to at home that morning? What did he/she eat? DID he/she eat? How does he/she do in school? Who are his/her friends. Then the questions go deeper: You're looking for the "why ?" of all of it.
Characterization includes traits and quirks, sure. But it's mainly about what each character thinks and feels, what they do, and the WHY behind all of it.
When you know your characters in that way, THEN they each come to a scene with their own histories and nuances. And then the answer about the "right" thing for each of them to do in that situation can come through in a real way.
Good luck!