i took one year of statistics in college nearly five years ago, so i might be a little rusty at this... but i think you can roughly approximate your ranking based on the following assumptions and equations
(1) assume grades fall on a normal distribution
(2) z = (x - u) / s; where x = your GPA, u = the mean GPA, and s = standard deviation
(3)ESTIMATE YOUR STANDARD DEVIATION!! s = (L - S)/4; where L = highest possible GPA, S = lowest possible GPA (note: since all Fs are unheard of, I simply put the lowest possible GPA as a D average, which makes sense, at least at my school where all As is about as rare as all Ds); instead of 4, you can use 6 if you are more confident about your L and S estimates...
(4) calculate your z, and reference the following table http://www.sruweb.com/~walsh/witte_z_table_1.jpg
; your z number should be a three digit number (negative or positive) with a decimal after the first number
If your z number is positive, you are ahead of the median!! look at the second column; it will tell you what percentage of students are ahead of you!!
If your z number is negative, you are behind the median!!
look at the second column, it will tell you what percentage of students are behind you!!
I hope this helps.. this is a rough, rough, rough approximation.. hahahaha we've made some huge assumptions here (like everything falls on a normal distribution).. many teachers have told me though that if you have a big enough sample size (i.e. class size), usually it works out that way... besides, most schools enforce a distribution, but it isn't always normal.. close enough, though
if there is some other more sophisticated way, i'd love to know..
math nerds of the world, unite!