Universities look at the courses necessary for the faculty you are applying for. It seems logical that RMC would do the same. You may need two courses that meet the prerequisites (eg. you need two of bio/phys/chem for sciences), but lets say you take all three. In that case, it seems they will look at your two highest marks. We would all love PE, peer tutoring, woodshop, drama class, etc. to be included in our average, but they never are (unless they are a pre-req, which they rarely are).
So to be clear, for example:
Prerequisites: You have:
2 of bio/phys/chem All 3
1 of english/poetry/communications english
French 12 got it
1 of math 12/pre-calc 12/calc 12 math and calc
1 of socials 12/ ancient civilizations 12 socials
Then, you will be evaluated on: The two best marks out of chem/bio/phys, english, french, best of math/calc, and socials. No grade evaluation done on courses which are not pre-req's.
Sorry for the long winded answer, its late here.
Jonathan