Where in Canada can I study for a High Course by myself and earn a credit by finishing an exam?

Hi everyone. I’m a Canadian citizen who is interested in taking a few High School credits in order to apply for university. I was wondering about the option of challenging for a credit:in other words, studying for the course by myself and taking an exam to earn a credit. I don’t live in Canada right now and I’m wondering what others know about challenging for a credit in various parts of the country. What would be the best way of going about it? Thanks in advance.