Which yoga positions are good for people with scoliosis?

Anything: Exercise will not change scoliosis in adults or growing children. That is a proven medical fact. Core exercises, though, are great for anyone with back pain or a tendency for it. So progress flexibility as you tolerate. Don't force motion, as your curve may block some movements. Enjoy!

How severe?: If you have severe medical problems get medically cleared. Yoga Back Poses: Bharadvaja's Twist (Bharadvajasana I), Big Toe Pose (Padangusthasana), Bridge Pose (Setu Bandha Sarvangasana), Cat Pose (Marjaryasana), Chair Pose (Utkatasana), Cobra Pose (Bhujangasana), Cow Pose (Bitilasana) & Crane (Crow) Pose (Bakasana). REF: http://www.yogajournal.com/poses/anatomy/back