The board of a school district shall grant high school credit in any course to a pupil enrolled in high school, but who is not enrolled in the course, who has exhibited a reasonable level of mastery of the subject matter of the course by attaining a grade of not less than C+ in a final exam in the course, or, if there is no final exam, by exhibiting that mastery through the basic assessment used in the course which may consist of a portfolio, performance, paper, project, or presentation. For the purpose of earning credit under this section, any high school pupil may take the final examination in any course. Credit earned under this section shall be based on a "pass" grade and shall not be included in a computation of grade point average for any purpose. Credit earned under this section may or may not be counted toward graduation, as the board of the school district may determine, but the board's determination shall apply equally to all such credit for all pupils and credit earned under this section shall be counted toward fulfillment of a requirement for a subject area course and shall be counted toward fulfillment of a requirement as to course sequence. Once credit is earned under this section, a pupil may not receive credit thereafter for a course lower in course sequence concerning the same subject area.