I suppose a corny answer would be to be yourself, as you would teach better if you feel comfortable. However if they are younger than 16 they may not treat you with the respect you deserve if they're rolling around on the floor in stitches at the way you look. Personally, one of my greatest teachers never cared what he looked like, I'm sure he actually wore the same suit for the whole 5 years that he taught me...but that was over 12 years ago and some kids don't seem to have been taught to respect their teachers these days...let us kno how it goes.