Where did the F, D, C, B, A, S, SS, SSS ranking system originate?
The F, D, C, B, A, S, SS, SSS ranking system is commonly used in video games to denote the performance of the player. It is believed to have originated in Japan, where it is widely used in arcade games and later in console and computer games.
I think "S席" may mean "Special Seat". In Japan at least 1910, "特等" grade was used for the top grade seats (http://parasiteeve2.blog65.fc2.com/blog-entry-1135.html). "特等" > "一等(first grade)" > "ニ等(second grade)", or "特等" > "普通(normal or basic)" were used past (these are also commonly used for seats grades even now). "特等" is abbreviation for "特別等級" = "Special Grade". So S may mean Special.