Nothing within this chapter shall preclude the board of directors of any district offering a high school diploma from establishing such additional course, credit, and test requirements as deemed desirable. A district may not adopt any policy which requires enrollment for either (a) a minimum number of semesters or trimesters or (b) a minimum number of courses in a semester or trimester which exceeds the enrollment time or courses necessary for a student to meet established course, credit, and test requirements for high school graduation.