Sun Salutation is not considered as asana because it is  dynamic. asanas are static. 

But this is an excellent  warm up exercise. It warms up each part of the body. Students should never skip it!  The important thing is each movement should be synchronising with breathing, and the breathing should be deep and slow.

In sun salutation  the movement should be flowing. As the movement synchronising with the breathing the body create heat and warms the body..

Never skip the sun salutation in your daily practice.