Maty Ezraty's reply:
I do not recommend an adjustment for this! Some beginning students are simply not strong enough in their arms or legs to practice Upward-Facing Dog. For these students, it is best to spend time in soft Cobra, with your ribs on the ground. In Cobra Pose, they learn the correct leg actions and build the necessary strength in the lower back to later press up into Up Dog. To strengthen their arms, spend time teaching Puppy Dog and Downward Dog. (Puppy Dog is a preparation to Down Dog, done at the wall. The torso is parallel to the ground and the feet are far enough away so that the body forms a right angle.) In other words, the student should be able to press into Up Dog on their own without an adjustment, or they are not ready and are best in Cobra.
Maty Ezraty is co-creator of the first two Yoga Works yoga studios in Santa Monica, California. A former YJ asana columnist, she travels around the world leading teacher trainings, workshops, and yoga retreats.