You’ll probably want to do more problems than just the ones Khan Academy offers, but it’s a great way to learn the content. I would recommend sending an email to your school counselor and assistant principal (those were the people that handled class schedules in my middle school, at least) and ask about what you could do to test out of geometry. At the end of the day, you’ll need somebody from your school to verify that you have the knowledge and skills, probably with a standardized test of some sort. Good luck!