If I were Harry, I'd realize that I wouldn't be bound by the restrictions on undeage magic. Since Dumbledore forced him to attend, he doesn't have to worry about being expelled. I'd also consider doing something like showing up, but during the Sorting walking up and telling everybody what happened. Then going up to Dumbledore and punching him in the nose. Then saying, "Now that I've publicly attacked a staff member, I won't ask for leniancy. I must be expelled. I guess I'll be going back to Xaiver Instutue. Goodbye magical world.