Look at your current seating chart. Identify the students who consistently misbehave. Move these students away from one another and use quiet, behaved students to block them from one another. Do not be afraid to move a disruptive student to the front of the classroom where you can keep a closer eye on them. Shaking things up can help disrupt current behaviors and puts the teacher in a position of control. As soon as students enter the room, have them stand in the front and explain why you need to make the change.
Some teachers avoid calling parents, but calling home can work to your advantage. When a student is disruptive, call and explain the negative behavior. Many times, parents willingly help and will establish consequences for the student at home. Cell phones can be taken away or computer time limited. For students, a parent's power to punish can be more severe than the teacher's consequence. When word gets out that you have called home, many students will curb their behavior because they do not want you to talk to their parents.
Many disruptions happen because students are given free time. Teachers need to teach from bell to bell. Start with an anticipatory set of "do-now" tasks at the beginning of the period. Students should come in and start the assignment immediately. Over-plan for each lesson so there is no chance you will finish early. Keep the class moving with various activities, limiting the chance for negative behavior. Never give students a free period or time at the end of the class to start their homework if you have a difficult group.
Establish clear rules and enforce them fairly. If the class consistently misbehaves, you can introduce some new rules. Let the students know what the changes are in writing and what will happen if they are not followed. For example, if a student has to be spoken to more than once in a class period, he will be given detention. Once you establish a rule to get control, you must follow through with it. Do not give another chance. Students respond to teachers who are fair and firm; they take advantage of teachers who they see as weak and inconsistent.