Your cooperating teacher has many years of classroom management experience. While you are still in the observation stage of your assignment, notice what techniques your mentor uses to control behavior and motivate students to stay on task. Note which methods you'd like to try for yourself, and which you might be uncomfortable with. Remember, your cooperating teacher has had many years to master the art of classroom management. If they're working, continue to enforce his rules and routines, especially if you are taking over the classroom partway through the year.
Students -- especially those at the middle and high school level -- will try to provoke a student teacher by testing her limits. This is especially true if you will be taking over as a student teacher in the middle of the year, versus in the fall. If a student begins to act inappropriately, it is critical that you immediately, firmly and authoritatively correct him. Start with a warning; if the behavior continues, inform the student that there will be consequences. Do not offer empty threats. If you imply that discipline will follow improper behavior, you must follow through with it. Otherwise, you will swiftly lose control of the classroom, and encourage the chronic misbehavers to continue, well, misbehaving.
Again, following your mentor's lead, enforce a set routine. Create a daily startup activity that students must participate in as soon as they enter the room. Do not allow for downtime. Be sure that you have enough material to keep your students on task during the entire period of study. As the saying goes, "Idle hands are the devil's playground." Students who are bored or unfocused are likely to cause disruptions and behavior issues.
In a perfect world, all students would behave like little angels, quietly completing their assignments and politely and respectfully answering the teacher's questions. Experienced teachers know that this is never the case. Know going into your student teaching assignment that situations will arise that you may not know how to handle. Try to stay calm. Yelling at a student, or losing your cool, will only escalate the situation, make the student feel defensive and make you look unprofessional. If you need help from your cooperating teacher, ask for it. Above all, remember that student teaching is a learning assignment; try to process each event -- whether good or bad -- as part of the journey toward becoming a qualified, successful, confident teacher.