In my ranking system the C ranks can fight 1,000s of normal humans so they are not that weak, even if S rank can fight 1,000s of them. A rank can only fight 100s of them. F rank is a normal human although there can be lower ranks for cripples and weaker species. And babies. They don't bother to get ranks though unless part of some funny gag. S ranks aren't even found on a lot of worlds so there might be one per every 10 or so planets at most. If they are on a planet they could probably conquer it if they felt like it, given that all the inhabitants are only F rank. The main heroes of one of my stories are S rank but there's higher ranks such as the one right above them called Legendary that one of their main opponent is.

That's some academy allowing studies to get ribs broken. :sun