Read Aadil Palkhivala's reply:
Dear Kwan Nyun,
The reason your students do this is because their hamstrings are tight. The hamstrings attach to the bottom of the pelvis, and when they are tight, they pull the bottom of the pelvis toward the knees, thereby tipping the pelvis backward. This causes them to end sitting on the sacrum. At this point, any attempt to do front bending or most sitting poses is actually dangerous, since it strains the sacroiliac joint as well as the lumbar vertebrae.
The first rule of teaching is to give students correct instructions and then give them a chance to make the correction by themselves. If that does not work, then you can help them with an adjustment. Finally, if that does not work, give them a prop. In this case, the correct instruction would be to tell them to place their fingertips on the floor next to their buttocks and lift the pelvis slightly off the floor. Then scoot the pelvis back a little so that the pelvis tips forward (a demonstration helps at this point). If this does not work, give them a prop to sit on, such as a firm, folded blanket or a block. Note that in this case, there is no adjustment you should make.
The real cause of the posture, however, has to be addressed, and that is the tight hamstrings. The safest pose to lengthen hamstring muscles is Supta Padangusthasana (Reclining Big Toe Pose). It is a rare pose indeed, the only one in which your students can effectively and strongly stretch their hamstrings for prolonged periods of time and yet not injure their lower backs or sacroiliac joints.
Recognized as one of the world's top yoga teachers, Aadil Palkhivala began studying yoga at the age of seven with B.K.S. Iyengar and was introduced to Sri Aurobindo's yoga three years later. He received the Advanced Yoga Teacher's Certificate at the age of 22 and is the founder-director of internationally-renowned Yoga Centers™ in Bellevue, Washington. Aadil is the director of the College of Purna Yoga, a 1,700 hour Washington-state licensed and certified teacher training program. He is also a federally certified naturopath, a certified <a href="/health/ayurveda">Ayurvedic health science practitioner, a clinical hypnotherapist, a certified shiatsu and Swedish bodywork therapist, a lawyer, and an internationally sponsored public speaker on the mind-body-energy connection.