Read Maty Ezraty's response:
You are correct in suggesting that there are many benefits to aligning the shoulder girdle in Plank Pose before lowering into Chaturanga Dandasana. It gives the student time to align the shoulders and to maintain the work as they lower into the pose. By learning Plank, beginning yoga students will build the proper strength and understanding to keep the shoulders safe in Chaturanga.
On the other hand, it's true that beginning students could jar their lower back and jam their elbows while attempting to jump into Plank. It's like landing with a straight knee rather than with a bent one. Beginning students can lock their elbows and jar the lumbar if they are not aware or strong enough.
Therefore, in beginning classes, I often recommend stepping to Plank and then lowering into Chaturanga.
Conversely, it is possible to jump into Plank with awareness, without jarring the lower back or the elbows. Any posture can be done with care and awareness or with harmful lack of attention. It may interest you to know that in the classical Ashtanga system, we jump right into Chaturanga. Therefore, the practice of Plank is an adaptation, but a useful one for many students.
Remember that there is no one correct way to do a pose. Look for the pros and cons of doing any given posture and make the best choice for your students based on what you see. Then make a choice that is well informed, compassionate, and rooted in ahimsa, or nonviolence. When you teach with this awareness, there is no right or wrong. There is only truth and what is good for the students.
Maty Ezraty has been teaching and practicing yoga since 1985, and she founded the Yoga Works schools in Santa Monica, California. Since the sale of the school in 2003, she has lived in Hawaii with her husband, Chuck Miller. Both senior Ashtanga teachers, they lead workshops, teacher trainings, and retreats worldwide. For more information, visit http://www.chuckandmaty.com.