Ideally you need your class to come up with a list of class rules and consequences and post this on a big piece of cardboard in your class. If the bad behaviour has started recently and you don't have any class rules, you can say 'I think that this class can work much better. In groups of 4, I want you to come up with 5 rules of acceptable behaviour'. Give them 10minutes. Then get a student from each group to write up their suggestions on the board. Ask the students to vote for each rule by raising their hands. Get the student with the best handwriting to write your class room rules on a big sheet of paper and display it in your classroom. Your school should also have some classroom behaviour rules. If you involve the students, then you can remind them of 'their rules' that they agreed on . Ask them to copy down the class rules in the front of their books. Don't allow any mobile phones or ipods. If a student's misbehaviour is constant, I suggest that you ask another teacher for advice who has taught that student before or that you ring their parents.

What do you mean about reasonable and else SPECIFIC classroom rules?

What is the age group? Are they foreign students? You will get better insight if you are more specific about the classroom's demographics . I mainly deal with instructing individuals between the 18-25 age range so my experiences may be different. Leadership is always a difficult task to undertake especially if your audience is younger. If you are dealing with unruly students there are methods that you can use before resorting to punishment. Sometimes trying to identify with the students before disciplining them can be effective. Maybe there are issues at home which are affecting their focus.