It happens sometimes. Horses are notorious for finding ways to injure themselves in what could be considered a "horse-proof" stall. Depending on how she heals, she may not have enough of a scar to worry about. Just keep it clean and moist (triple antibiotic ointment works wonderfully) and it will heal up fine. Keep checking it for any kind of heat, swelling, or odor so that you can get on top of any infection if it happens.
As for the halter class, they judge is not supposed to discriminate against scars or injuries unless they are extremely disfiguring (and possibly not even then, I'm not positive).