Keep the lessons moving. It is imperative that the children feel engaged; otherwise they will lose concentration. Do not lecture for the entire period but instead plan at least three activities that relate to the information taught in class. Not only will this keep the students calm and focused but may help them retain the information better too.
Stand next to disruptive kids. If a child is causing a disruption at the back of the class, stand next to him and supervise him working while you continue to teach the class. This should cause him to calm down a bit but will also allow you to see if he is having difficulties with the material being taught.
Allow the students to teach. This is a common strategy for controlling the classroom. While teaching a subject, assign students to a different topic of that subject and ask them to research for 10 or 15 minutes and then come to the front of the class and deliver a short lesson. If you feel a particular student would be anxious doing this, give her particular attention and help her along during her presentation.
Encourage the students. When your students do something well or kind congratulate them. This reinforces positive behavior and can cause a change of attitude. This extends to outside the classroom too. If you see them in or outside the school be friendly and affirmative. If they have personally achieved something outside of school congratulate them on that too.