“Adult,” for purposes of this section, means a person 18 years of age or older or a person who is not concurrently enrolled in a regular high school program. However, persons 18 years of age or older who are concurrently enrolled in a regular high school program are not adults for purposes of this section. Adults enrolled in any classes maintained by high school or unified school districts or county superintendents of schools pursuant to Sections 52300 through 52330, 8530, 8531, 8532, and 8533 shall be reported separately, and the adult foundation program shall be used in computing district apportionments.