1.    Except as otherwise provided in this section, each student shall enroll in at least four units of high school work in each grade from nine through twelve.

2.    A student in grade twelve may enroll in fewer than four units of work, provided:

a.    The student requires fewer than four units of work for graduation; and

b.    The board of the school district has adopted an alternative high school senior curriculum plan.

3.    An alternative high school senior curriculum plan becomes effective if:

a.    It is adopted by action of the school board; b.    It contains specific criteria under which a high school senior may enroll in fewer than four units of work; and

c.    It has been submitted to and approved by the superintendent of public instruction.