The meaning of this word can be difficult to understand, specially when you are in a classroom, in front of fifty teenagers and you are the teacher.
From my point of view, discipline is related with your skills and experience as a teacher, I think that it should be implicit when you get your students attention, when you engage them in your class. It sounds great, but the reality is that you always have students who have all kinds of thoughts, behaviors even with many external problems, and I think the ability to deal with situations and disruptive students must be part of your training as a teacher.
All teachers sometime have had some problem with one student in the middle of the class. That is a terrible situation, what should you do?. If you don´t say anything it could be an example for the rest of the class to be rude with you. If you scream maybe the hole class could see you as an ogre and the relationship between you and the enviroment in the classroom could deteriorate.
As I say before, as a teacher you have to be ready to deal with all kind of situations and students in your classroom, remember, to get into a backtalk you need two people. We have to realize that time goes by, new generations are not the same of our generation, the way to teach should not be the same of twenty years ago and the concept of teacher-student relationship is different now.
I think discipline has to be an agreement between you and your students, an agreement of mutual respect, which involves a strong commitment of you as a teacher to find appropriate tools for your students to learn in the best way, to respect and motivate them to learn, and a commitment of them to take advantage of their opportunities.