The first thing to do with the essay is identify the various social classes presented to the reader in the book. The story presents readers with four distinct groups. They are Lunars, humans on Earth, cyborgs, and robots. For the purpose of your essay, I recommend picking two groups. This will cause your essay to be much more tightly focused. Of the groups mentioned, I would pick the humans from Earth and the cyborgs. Cyborgs are definitely treated as second-class citizens by most of the humans, and this gives Cinder's family an excuse for abusing her in a variety of ways. Cinder is fully aware of the social stigma of being a cyborg, and that is why she hides this information from Prince Kai.

The writing prompt appears fairly wide open, so you can discuss various angles of how the social class distinctions affect the overall society or Cinder herself. One specific point to make is that cyborgs are viewed as less than fully human. They are objects of sorts, rather than subjects. This is why Cinder and other cyborgs can be owned and sold, in processes very reminiscent of slavery. This opens your essay up to discussing how Cinder and her fellow cyborgs are dehumanized because they have machine parts. If you so desired, you could relate that to modern times and people with prosthetic limbs.