Headstand is called the king of asanas.  For me it is the most important asanas in yoga!  If the teacher is not teaching headstand in the class   .. for me  in my opinion he is not teaching yoga!  I teach headstand in my beginners class.  You should be in the headstand at least 5 minutes before attempting advanced variations.  Benefits from headstand are  so much!  If you have high blood pressure or neck pain don't attempt headstand!  .. Women if you have period it is better not to attempt this posture!   The benefits of the headstand are so many. It calms the mind. Give fresh blood to the brain, Increase the memory etc