R

Most colleges us the letter "F" to grade a course that you have Failed to pass. CMU however, uses the letter "R", presumably to denote a course that you will have to Re-take. An alternate theory suggests that they are telling you that you are Retarded, which really isn't very nice of them at all.

Coincidentally, CMU also uses the letter "R" to denote "Thursday" on class schedules.

It also happens to be the 18th letter of the alphabet.