re: What do high school AP classes do?

It gives you the opportunity to earn college credit
For example if you take AP English or AP Histroy you could test out of some fr college courses you would ordinarily have to take. The exams you take at the end of school would determine whether or not you could earn college credit.
It allows you to skip the first english.

Plus your ACT and SAT scores, you can take exams summer before you enter college to test out of courses.