Even for the best teachers, occasionally, a student will act with complete disrespect. An outstanding teacher will model a positive response. A student may yell out repeatedly in class every single day. An outstanding teacher will model a positive response by staying calm and using appropriate language. A student my get out of his seat or storm out of the classroom. An outstanding teacher will model a calm and respectful response.
The examples can go on and on. Kids may make poor choices when they reach a boiling point. The teacher's reaction will help determine what happens next. If your response is unconditionally positive, the results will be positive. If you let your emotions control your response, or you meet the student at his elevated emotional level, your response will most likely be negative.
I have seen a few examples of incredible patience over the last few weeks involving some quite difficult situations. Kids with truly stressful situations and inappropriate actions met with grace and dignity. As challenging as it may be in the heat of the moment, these positive examples of responses are always better than trying to "win" a tough situation with an emotional student.