In Ontario, to use your example of a "C" classified shooter, only scores of 35% to 74.999% will count towards their classification (5% below to 15% above their current class). If you shot 6/6 classifiers at 90% and had excellent match results to back them up, then you could appeal your classification to the BoD and will likely be moved to a more reasonable classification. You can move down in classification, but it would be a gradual decline as any score more then 5% below your classification will be discarded.