When wearing a scoliosis brace, are you able to wear anything on your shoulders like a bookbag?

Yes. Yes, a book bag can be worn over a scoliosis brace without any adverse sequelae. The scoliosis research society has studied the issue of backpacks and scoliosis and have determined backpacks do not cause or worsen structural scoliosis.
You can, but not. Recommended. If you hold it on the side of convexity, then you can cause increase pressure on nerve on the concave side. If you wear it on the concave side, you may cause micro-trauma leading to further curvature even with the brace. Avoid if possible. There are weight distributing backpacks that cause the weight to distributed off the shoulders and on to waist, consider that or rolling bag.