Former camp counselor for ages 10-12.
(Note it's 2 AM, so sorry if things aren't too coherent.)
Best of luck! and Best wishes for all those kids!

EARLY STAGES & MORE PROACTIVE THINGS - When we had bullying issues at the first notice or suspicion of bullying I would scold the group -- bullies and victims, just because often the victims respond with behavior just as negative -- but also notify other counselors and my unit leaders (perhaps other teachers and an administrator in your case) and let them know that they've had their warning and to keep an eye out.
- Depending on the victim's personality and everything, I may ask them how things are going. If they don't explicitly say anything about the issue, I would maybe pry a little further and ask "are things going all right with [so and so]?" or if you don't want to use names say "classmates".
- Always ask kids individually how they're doing or how their day is going -- individually, not just "so how was everyone's weekend?". It emphasizes that you care about them and I've found that then they trust me more and come to me with questions or problems sooner. Make them feel like they're always safe coming to you with any issue. I would literally just say aloud, "remember you can always come to me or another counselor[teacher] if you need anything or are having any problems!" probably every other day if not once a day to my campers. Do this all the time, not just when you suspect bullying. I swear it helps.
- In addition to talking, I've found moving seats or minor punishments are often more useful than a harsh one. Put the victim with someone whom you think he may click with, and the bully with people whom he gets along with but not his/her best friends.
- Also super positive reinforcement for positive behavior, like "hey it's really great that you helped [so and so] do this, it shows that you care about others and that's really great", or "hey, it's really great that you did [this], it shows [this great characteristic or motive]", if a bully feels better about him/herself than they seem to be less cruel to others. When victimized kids feel better about themselves, they're more confident and less likely to get as hurt by bullying. But I do this to everyone all the time, and excessively to bullies and kids who are bullied.
- I've listed some things that I would usually say to my campers and twisted more to a teacher/student discussion, but they're things that have worked for me before.
TALKING TO THE GROUP
- If anything continued I would talk to the kids all together, usually went something like:
- "I'm very disappointed with the behavior I've been seeing. [List some.] It's unacceptable. You guys are old enough to know that. You don't have to be best friends, but as people you need to have some respect for each other. For now, I'm not going to ask you to explain exactly what has been going on unless you want to, but don't think that I haven't been noticing. The younger kids all look up to a lot, they mimic your behavior and your image and your language, you guys need to be good role models for them -- and I know that you all know that [whatever behavior, name calling, pushing, etc] and bullying is not something a good role model does. I'm keeping my eye on you guys, and so are the other counselors[teachers] so don't think I won't hear about any incidents. You can always come to me or another counselor[teacher] if you have a problem. If someone is negatively impacting your day, don't go negatively impact theirs, come find a counselor (/teacher)." I've found this speech covers the whole shebang pretty much.
*TALKING TO THE INDIVIDUALS
- ** Let the victim know that he/she should feel safe at school and that you and the other adults are there to protect them, ask what you can do to help. Let the bully know that everyone should feel safe at school, and don't yell or scold excessively at the first discussion but express your disappointment as well as concern, ask what you can do to help. - After a group scolding I would ask the kids individually what was going on.
- *[things to say when talking to the victim] *
- Often I've found the kid being bullied doesn't want to talk about it. So I would ask, "do you feel safe?" and remind them that everyone should feel safe [wherever, camp, school, work, etc]. If they say no, let them know that all the other adults know about what's going on, and they won't tell their students or announce it to the world, but they're aware and ready to help out.
- "So I've noticed... [whatever] and it's really worrying me. What's going on?"
- "How come you didn't let me know what was going on right away?"
- "Hey, I'm here to protect you and help you out, that's my job you know -- of course I want you to learn [whatever] but more importantly, I want you to be safe and happy here, okay?"
- "How do you think we could solve this problem?" or "what do you think would help?". "I'm very sad to hear that it's been so rough that you didn't want to come to school, so how can I make things better? I want you to want to come to school"... later on if bullying is still occurring or not simmering down as fast as you would like, "What can I do to help you?". (Sometimes kids have genius answers to questions like these.)
- "Just because [so and so] did this, doesn't mean yo have to respond with [whatever]."
- - *[things to say when talking to the bully] *
- "So I've noticed... [whatever] and it's really worrying me. What's going on?"
- The "everyone should feel safe here" talk, but also say "if someone else is upsetting you to the point where you [whatever] to them, well that isn't fair to anyone right? You, them, me. What can we all do?"
- "I know you may not feel like [whatever] was exceptionally hurtful, but not everyone perceives things the same way so something that could've been a joke to you may not be funny to everyone."
- "How do you think we could solve this problem?" or "what do you think would help?"
- "I'm very concerned with how big of an issue this has become, so how can I make things better?"... later on if bullying is still occurring or not simmering down as fast as you would like, "What can I do to help you [control your negative behavior/reach your potential/etc]?"