Working with groups of people is never easy. When you add in the differences in ages and that you don't see each other on a daily (or sometimes even weekly) basis, it gets even more difficult. Here are some resources that might help.
What is a group and how do they become a community?
Stages of Group Development
Setting boundaries and norms in the group
Dealing with Problems
Bullying (description & prevention)
Bullying Prevention - from the American Camp Association: An article with tips and more links
Girls and Their Frenemies
Making a contract with children with misbehavior
Building Your Community
Leadership in Community: A Dance of Mutual Love
The Flow of Group Life (from EfM)
Stages of Group Development (from Sewanee)
Observing Group Process (from Sewanee)