The marks that matter are your grade 12 level marks (aka 4U/M in Ontario). So if you take a 4U/M course in summer school, that can count towards your admission average and their considerations for early offers. If you take a grade 11 (or other) course, then it will only count towards them deciding if you're worthy of an early offer.
ElectronicInfo.ca has the information for required courses and mark guidelines for all Ontario programs. For McGill or any other programs outside of Ontario, consult the university websites.
I believe Economics will require English and a math or two at the 4U level, and then enough 4U/M courses to make up 6 in total. Since it's under Arts, it may just require English, but you are going to want to have some grade 12 Math under your belt.
For marks, 85% in grade 12 would get you in anywhere for Economics except maybe McGill and Queen's; for those two you might have to bring it up a little bit.