You dont, except if you pursue a career involving mathematics. If you do however, you can learn it then, and not waste your time on it now.
Truth be told, the reason why the gov keeps you in Prison School until youre about 18 is because if you graduate and start working fulltime before then, you DONT pay taxes on your earnings as a minor.
Gov dont want that, so they keep you confined in mass indoctrination until youre 18 and convincing you that you have to go to post-secondary school right after that....effectively putting you in debt for the next 25 years, guaranteeing your slavish services until then to keep the Machine going strong.