Every single morning, I am blessed to get up and go to a job that I love. Many people actually laugh outwardly when I tell them that and then tell them what I do. I teach high school. No, I do not live in a fantasy world and yes there are teens that disrespect every person that they come in contact with on any given day. However, there are ways to deal with these disrespectful teens that allow both you and them to move forward and get on with the day.
The very first suggestion that I have is to be sure you are treating the teenager in question with respect. It does not matter what they are doing, you are an adult and need to act like one. Besides the fact whether you are a parent or a teacher, for many teens, you may very well be the only positive role model they encounter. It is up to you to model for them the behavior you want them to exhibit.
Another thing you learn quickly with all teenagers is to not take anything personally. Often times, the same teen that you had a great talk with the day before will push past you in the hall and act like you are the enemy. A great deal of the time, it has nothing to do with you. If you allow yourself to dwell on it, you do not have time to deal with the good parts of your day. Chances are you will find out later that it had to do with a peer issue or a perceived tragedy in their life.
Certainly, you can not allow a teen to continually disrespect you. It is now time to have a plan of attack. The first thing is to never confront them in front of anyone else. They will have no alternative but to increase their level of disrespect. Teenagers are at a difficult age and they need to keep their dignity in front of friends and even siblings. Make the conversation private and more importantly, make it calm. There is no need to yell or accuse. Let them know you can't tolerate their behavior, identify the expected behavior, and outline the consequence should they choose not to show respect in the future. For most teens, this may be enough to get your relationship back on track.
If for some reason you are dealing with those teenagers that just will not let up, be prepared to follow through with the consequences. Nothing is more important than letting them know you were not kidding with them. It may be difficult, but it will be worth it in the long run.
Remember, teenage years will not last forever. Use your knowledge of the particular teen and your common sense and eventually, things will probably work out for everyone involved. If they don't, it may be time to seek professional assistance. No matter what you do, the best strategy I have ever found for dealing with disrespect is to care about them. Most of the time, that is what they really want anyway.