When I applied they had this optional application thing you could fill out which outlined your extracurricular activities and allowed you to explain why some (if any) of your marks weren't great (death in the family during the school year, etc).
I was in a similar spot as you (I think I finished high school with an 81 point something). I filled out the form in great detail and proof read everything to make sure there were no spelling or grammatical errors. I sent it in on time, and in 2008 I graduated with an Hons B.Comm & a Minor in Economics.
Based on this link:
http://registrar.mcmaster.ca /liaison/?p=50 it seems these days they'll mail you an optional application if your grades aren't high enough to get you in on their own.
If they send you a form, do as I did. If not, then let me be the first to welcome you to The Degroote School of Business.
- Alex McColl
B.Comm 08