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I recommend finding out more about your student’s scoliosis. Is it a C curve, S curve, isolated in the thoracic or lumbar region? Also take note of the directions of the rotation of the spine. Is the student experiencing discomfort only in Savasana, or in other asanas as well?
When looking at the rotation of the spine, you may be able to see that some asanas help unwind the muscles on one side, while the same asana may feel like it is having a jamming effect when performed on the other side. So your student may need to do the pose on each side slightly differently. For example, in twists, he or she may be able to twist freely to the left but may need to work on just creating space along the side of the back when twisting to the right. Of course, this will take a little time to figure out, but if you can help the student develop this type of awareness, it will help in all of the asanas.
Since I don’t know anything about the nature of your student’s scoliosis, I can only tell you a general approach. In Savasana, you will find that, due to the spinal rotation, some parts of the back may bulge and press into the floor, while other regions of the back are concave and have no contact with the floor. It can be helpful to fill in the gaps between the concave portions of the back and the floor with a soft support, such as rolled blankets or thin towels. The prop should not push into the body but should just provide a sense of support so that portion of the body can rest while the two sides of the back receive the same sensory feedback from the floor.
A common scoliosis pattern is the rotation of the thoracic spine to the right, which can cause the right side of the back ribcage to shorten and bulge and the right shoulder to roll forward. If your student is feeling strain in the left shoulder in Savasana, it may be caused by weight falling from the right to the left side. In this case, you can try supporting the left side of the upper and/or middle back.
Start with a thin support; a little can go a long way. You may also need to place a thin support underneath the right shoulder blade to wedge it inward toward the midline. Support on one side of the body may require compensatory support somewhere on the other side. Be willing to adjust several times until you find what makes her or him comfortable—and ask your student for feedback regularly.