The Legislature recognizes the importance of assuring that all persons who graduate from Nebraska high schools possess certain minimum levels of knowledge, skills, and understanding. Beginning in school year 1987-88, each high school student shall complete a minimum of two hundred high school credit hours prior to graduation. At least eighty percent of such credit hours shall be core curriculum courses prescribed by the State Board of Education. The State Board of Education may establish recommended statewide graduation guidelines. This section does not apply to high school students whose individualized education plans prescribe a different course of instruction. This section does not prohibit the governing board of any high school from prescribing specific graduation guidelines as long as such guidelines do not conflict with this section. For purposes of this section, high school means grades nine through twelve and credit hour shall be defined by appropriate rules and regulations of the State Board of Education but shall not be less than the amount of credit given for successful completion of a course which meets at least one period per week for at least one semester.