Grade 13 was not optional if you wanted to go to university.
Grade 13 was eliminated by the Mike Harris government to save on education spending. It had nothing to do with giving Ontarians an unfair advantage (lol). In fact it was a disadvantage to be a year behind everyone else in the country.
They didn't just scrap grade 13. They planned way ahead. They changed the curriculum at grade 6 (I was in grade 7 when the new curriculum hit the grade 6ers). When I graduated 13, my grade 12 cohorts were just as educated.