Set the example. Remember that you are the teacher. It is important for you to be like a "superhero" figure in their eyes. Remember that your students look up to you and will thus try to mimic your dispositions. If you are rude or inappropriate, they will have an inappropriate model for their behavior. It is vital that students see you as a person with confidence, so that they follow your lead, and feel comfortable trusting you. Students, of all ages, need someone they can lean on, look up to, and be able to trust. Have well-defined consequences. Set specific consequences for breaking the rules. Decide what those consequences are and then implement them consistently. Your consequences should follow a procedure that starts with a non-verbal signal (such as just looking at the student), to a verbal signal (asking the student to please stop talking), to a verbal warning (if this continues there will be consequences), to the implementation of the consequence. The consequences are up to you and depend on the program of the school. Many schools have a detention system (students do despise detentions), or perhaps writing lines, or sitting away from other students.
Belinda