I have come to realise that the importance of making a good first impression is not just related to job interviews. It also applies to teaching. What happens during any first lesson I have with a new class can set the tone for months ahead, and the impression created during that first meeting can be extremely difficult to change.
This fact can make teaching very problematic. The chaos that often accompanies the beginning of a new school term means that it can be difficult to always maintain a confident and competent manner. It is vital that the teacher gives this impression from the beginning though. Students are highly skilled at sniffing out weakness in a teacher and don't suffer fools gladly. If the class have decided that they are being baby-sat by a disorganised idiot, it can take a lot to change this opinion.
I remember last year being given a last minute substitution and arriving at a class completely unprepared. I managed to keep the class occupied for the hour, but it was obvious that I was making it up as I went along. I left the class feeling embarrassed by my lackluster performance and feeling relived that at least I wouldn't have to face that particular group again. It turned out thought that I was to inherit this class and it took weeks to change the prevalent attitude of, " her comes Mr confusion". Some of the students continued to see me that way until the end of term.
Now I try and treat these first classes like job interviews.