A great place to start is the Ontario Federation of Teaching Parents. This site is great for an overview of the legal issues in home education in Ontario, lists of support groups organized by areas, and more.
The Canadian Homeschooler is a wonderful blog for finding resources from Canada. You can easily look up information by clicking on the subjects at the top of the site.
Related to High School:
Ontario Secondary School Literacy Test
What you need in Ontario to graduate high school
Home School High School - Check the rest of this site as well for University Admissions.
