Whatever happens in a lesson, it is never personal. Once you are done with the lesson, no matter what happened, say "good bye" or "see you at the next lesson" calmly and with a smile. If you bump into your student in the hall, don't brush him off and don't scold him again. What happens in lessons stays there. You don't want your students to think that you hate them, that you "just don't like them", that you have it in for them in some way. Every negative comment in lessons must be the result of a musical problem and it is very important that the student understands this. Don't slam the door after them, don't start yelling at them when you meet them in the hallways of the school or at the bus stop and remember to greet them and to let them go with a smile. I come across students that I personally dislike now and then, but there is no reason for them to know that. You have to stay professional.