I'm Canadian. I finished Grade 12 mathematics (and took the "school leaving" exam in that subject) when I was in Grade 7. Simon Fraser University (where I attended) has a special entrance category for "secondary student with superior academic standing" to take "one or two courses" while still attending secondary school.

I was admitted under this category, and when my "one or two" courses ended up stretching into half a degree... well, as long as I was getting mostly A+ grades, nobody was going to step in to say that I wasn't allowed to take any more courses.

As a practical matter, I'm sure it helped that my father was a Professor of Chemistry and a widely-respected member of the University Senate. When my admission required approval from the Dean of Science, his response was "no problem, I'll see him at lunch tomorrow". I like to think that I could have gotten in without that connection, but having someone on the inside who knew how the system worked absolutely made things run more smoothly.