There are two choices when it comes to classroom behavior that teachers can make:
1) They can be reactive and wait and see how a class behaves and then add some discipline structure if things don't go well
2) They can be proactive and have a classroom management plan in place from the beginning.
Don't wait and see if your class behaves badly before you decide to do anything about it. By then it may be too late.
Have a plan set up ahead of time and then enforce it as necessary. Making up something on the fly is asking for trouble.