How do I do a 4th a-level outside of school?

Hi! I've asked this question on reddit, and instead of telling me how do so I've just been told I'll regret it. My school doesn't allow people to do 4 a-levels unless they average 8s and 9s, and I average 6s. I want to do politics, but it isn't running at my school due to a lack of interest. So I want do it as a 4th a-level outside of school, but I don't know how to go about it. How do I do this?