Be a good and neat dresser, be businesslike, and be friendly. Be prepared for the unexpected. Only in rare instances is it appropriate to punish a whole class for the actions of a few. Never say anything to a student in front of a class that you would not say in the presence of their parents. Never, never humiliate a student in front of others.
Students have plenty of buddies.
Don't be afraid to apologize. Use the telephone. Let the parents work with you. Never argue with a student in front of the class. The odds are 25 to l that you'll lose.