In addition to the already posted, I have found report cards similar to our "históricos".       Reading them, it seems like they also may use a system of letters and numbers explained beforehand, something like the following: 

  http://www.mereggie.com/Grade-School/report-cards.html
"Promotion Policy: Promotion will be attained if a student maintains at least a "D" average with no more than two subject below "D", both of wich cannot be Math and English and none bellow "E"

   Then comes the notes (graded by letters), and the yearly average grades represented by numbers.

   Or it ends in general averages, given in numbers either.
 http://www.ghmchs.org/thisweek/photo-listing_files/Reportcard.jpg

   So the overall average is in the very end of it, the student knows than if he aced it or screw it by seeing the general average (grading).