I am working on this for playing in historical settings and, after a lot, lot of painful failures, I came to the conclusions that social classes are dependent on their relevant folk/culture.
Example 1:
Frank society had from bottom to top:
Wargs (cast out people, they are considered animals, a warg being a feral beast turned mad that attacks human settlements... most of the time they were wolves. Standard wolves - the ones staying far from settlements - were called ulf(en))
Slaves/servi (roman style ones)
Slaves/thralls (german style ones)
Leti (latin name, members of other german tribes that have been conquered)
Ingenui (latin name for free men - most franks richest gallo-romans and members of other tribes that have joined peacefully the frank confederation)
Meliores (the nobility, people bcame "noble" through education, wealth and honor... consider the meliores as the top of ingenui... nothing may prevent an ingenuus to become a melior)
Merovingian family (kings, queens, princes, princesses)
Inside ingenui and meliores, there were separate classes, some of them stading "out of" or "above" the social ladder. Basically, they were people that played a part in the administrative ladder or who had accumulated enough honor to be considered extraordinary beings.
Thengn: local leader, village head. chosen for his wits and valor.
Rachimburg: elder who counsels the thengn during trials.
Antruscion: Member of a "trust", a group of wxariors bound together by oath of fidelity to a rich and powerfull man. They are honored and respected more than anyone besides kings.
Paladins: palace officers, often antruscions of a king or queen.
Nutriti: people fed and educated at the king's table. Meant to be a core of administrative officiers. often very loyal and obedient.
Besides this ladder, add the religious one.
Example 2 : Byzantium
Slaves
Free men
Soldiers
Nobility (i.e. army officiers)
Emperor
And that's all. Yet there are some specifics but public officers are protected by the law, they are not a "social class" in the stricto sensu.
When you formulate social classes, I'd recommand tying a social system to a given folk/culture and see how you can link some benefits to it.
For example, if you say nobility is the core of warriors in a given culture, give nobles a +1 ST stat bonus and hand them a -1 to RE.
Other than that, don't consider taht you can't access sciences and lore when you are a peasant. Taht's wong. PEasants knew far more about healing herbs, seasons, crop growing, flora and fauna taht rich town citizens did. Even nowadays, I can find in France people that are able to cure some illnesses just through popular knowledge (for example, aspirin was devised from plants found in marshes, as was velcro (it comes from bardane).
Rich people may know more about reading, advanced maths (not mandatory), trading, influence and so on where peasantry would know better about growing and living things, crop tending and healing.
My 2 cents.