Do my aunt and uncle have to have guardianship of me?

Yes, they do. They have no authority to sign you up for school, to sign any permission slips for school events, to consent to your medical care, or anything else, unless they are declared your guardians.

You keep asking this. Until you turn 18 only the courts can make them your legal guardian. You cannot do this without your parents going to court and signing over guardianship

Once you are 18 you no longer have a legal guardian as you are an adult. So once you turn 18 you say you are an adult and do not have a legal guardian. But until your 18th birthday only the courts can assign guardianship if your parents want to transfer guardianship to another relative

Once you turn 18 you tell the school you are independent and do not have a guardian as you are an adult.

Yes they need to be your guardians to enroll you in high school. Establishing guardianship is very simple. A lawyer should be able to draw up the papers pretty quickly. High school does not care you are "almost 18". You either are 18 or over or you are not. You are not.