at the end of a stage, you are ranked based on a combination of your ring total and ending score. (the time remaining adds to the score.) then if you clear the stage without dying you get one rank higher. so you must first do well enough to receive an A rank, and also not die, to get an S. i received my first S rank when i went for the "Join the Ranks" online achievement, however it didn't count, so it seems you must get an S on a regular level. challenges and online ranking levels (found in the title menu under Xbox Live) don't work.