They know where they need to be and when. Good teaching is also about style. Effective teaching is not about being locked with both hands glued to a podium, or having your eyes fixated on a slide projector while you drone on. Good teachers work the room and every student in it. They realize that they are the conductors and that the class is their orchestra.
All students play different instruments and at varying proficiencies.
A teacher's job is to develop skills and make these instruments come to life as a coherent whole - to make music. This is very important, good teaching is about humour. It's about being self-deprecating and not taking yourself too seriously. It's often about making innocuous jokes, mostly at your own expense, so that the ice breaks and students learn in a more relaxed atmosphere where you, like them, are human with your own share of faults and shortcomings. Good teaching is about caring, nurturing and developing minds and talents.