Define your terms. This is the most important aspect of any essay, not simply essays about social class. What do you mean by social class? How many classes are you covering? Will you be conducting a comparative study across countries? Make sure that you explain what exactly you are going to cover in the introductory part of your essay.
Decide which class theories will be your focus. A Marxist reading would say there are two classes: the "employer" class and the "employee" class. More orthodox theories divide society into three classes: the "working" class, the "middle" class and the "upper" class. It is best to refer to more than one theory, although base your work on one.
Be analytical in your approach. Within different social classes, there are many different subdivisions. A more nuanced essay will be a stronger one. Strengthen your essay by referring to the "artisan class" and "skilled or unskilled laborers" rather than simply the "working class". Class labels are very broad, and shifts in society often come from subdivisions.
Be aware of shifting class boundaries. Society is inherently fluid, with upward mobility being a agent of change within social classes. A profession which was "working class" one hundred years ago may be regarded as "middle class" now, and vice versa. Never assume that class boundaries are static. The rise of the "middle class" after the Industrial Revolution greatly changed the developed world.
Avoid generalizations about class. You would never do the same about nationality or ethnicity, so do not make assumptions about class. Use "working class" as a term rather than "lower class", which can be derogatory.