Some of this is less about answering an interview question and more about how to practice discipline when you have your job:
-The best discipline is a good lesson. Students who are engaged do not cause behavior problems.
-Students don't sign up for band to listen to you talk, they sign up for band to play their instrument. They tend not to misbehave when they're actively playing and you should do as much of that as possible.
-Be on the lookout for ways to reward good behavior as much as you look for behavior you need to redirect. It's easy for people to be discouraged when they're only recognized for the things they do wrong.
-When students are acting out, stand next to them for a bit, say their name, or give them "the look" to redirect the behavior. These are gentle reminders to them of your expectations and you should inform them this is how you warn students.
-If more is required, write their name on the board and ask them to see you after class. This is a more formal warning where you will calmly explain how their behavior didn't meet your expectations and that you will be contacting their parents next time. Write down that you spoke after class and any details you need to remember when you do have to call home.
-From there it escalates to a parent phone call and then detention.