Should I include my (unofficial) class rank in my SOP? I'm applying to a top program, and one thing I'm not super confident about is my GPA. It's good (it satisfies the course's requirement) but not stellar. Even my referees probably won't say amazing things about how I ranked in their classes (I got A's in all of them while some classmates of mine got A+'s).

However, because all of my class participates in an exchange program, at the beginning of second year (I am studying in Europe, so our bachelors take three years) a ranking is released including a score (GPA + number of credits) and the university where we'll go on our exchange. I ranked pretty high on that one (top 6%), so I think that including it somewhere in the SOP couldn't hurt.

However, it's not an official class rank (my university doesn't do those), and I'm afraid it would come across as a little desperate (which I absolutely am). What do you think?
In general, I think mentioning unofficial rank is fine. You could simply write:


  Note, though my university doesn't officially publish class ranks, I learned that my 3.6 GPA corresponds to the 94th percentile.


The only trouble is that they might not believe you; it would be better if you could get a professor to mention this fact in their letter. 

Your case is a bit different, though. You don't know what your final class rank is (even unofficially); all you know is that two years ago, you were in the 94th percentile of a subset of the students after weighting for the number of credits completed. There is no concise, accurate way to describe this that doesn't appear desperate*, and you wouldn't want to be accused of trying to mislead them. So, I would skip mentioning this altogether.

*Actually, the one way I can think of is maybe you could present it as an award? "I was ranked in the top 6% of applicants and was therefore allowed to study abroad in Jamaica?" If study abroad is a competitive process that you "won", it may be fair to mention this.