@TechnoWizROKYeah, I'll just clarify the 'S Class' meaning real quick, and leave it to the other guys for CS Critique.
S-Class isn't a 'normal' rank. It doesn't necessarily mean that someone's more powerful than the other members of the guild, it means that they're a member trusted by the Guild Master, and that in the absence of the Guild Master they're in charge. It means they're also trusted to represent the guild wherever they may be, and to handle special missions. S-Class is granted by Guild Masters under select conditions, usually including a trial, and is almost never offered to people who wouldn't make good leaders within the guild anyway.
So, being an S-Class mage really only means that you're an 'officer' of whatever guild you're a part of, and isn't an actual denotation of power.
A good example of the distinction is comparing Mark Zerrit, an S-Class of the dark guild Iron Enigma, to some of the A-Class members of Phoenix Wing. Many of them could beat him in a fight, even if he is S-Class. This is mostly because Mark was selected for an S-Class for his friendship and reputation with his guildmates, rather than his sheer magical power (Though he's not a pushover by any means). There was also Luna (Who should only be A-Class now because she's not in a guild), who possesses an impressive talent for magic and could probably be a serious combatant for almost anyone, but wouldn't have been S-Class if she didn't also have the ability to at least pretend to be a real dark guild member when out in the field to represent the guilds interests (and maintain a certain positive relationship with the other members of Iron Enigma). Meanwhile, Solux, who is a powerful member of Iron Enigma as well, will never make S-Class because he is too volatile and dangerous.
That should be a decent explanation. (I chose members of Iron Enigma for my examples because I run that entire guild myself, so I know their guild master's thought processes on the matter, but I'm pretty sure it's something pretty similar everywhere).
