Can an 18 year old enroll herself in high school Alabama?

No, an 18-year-old cannot generally enroll in high school in Alabama. The compulsory attendance age in Alabama ends at age 16, and while there may be exceptions for students with extenuating circumstances, a typical 18-year-old would be considered too old for standard high school enrollment. They would need to pursue a GED or other alternative path to earning a high school equivalency.