In South Australia (and possibly the rest of Australia), grade 13 is another name for re-entry high school, in which an adult student can study year 11 and 12 subjects to complete and/or top up their high school certificate.

The name "grade 13" in this case is essentially worthless, since it's really just grade 11 and 12. The term only serves to confuse people who have no direct links to re-entry students, and therefore completely misunderstand the concept.

When I first began high school, I intended to take year 13 for completion's sake. Now I've almost finished year 11, and will (hopefully) get my high school certificate at the end of next year. Excluding tertiary education, further completion is not necessary.