How can I integrate a new system to a design scheme I previously created?
Would it not solve your problem to redefine the GRADE_LEVEL table such that it contains either the grade of a course or the level of the course (which ever applies)?

The rows in GRADE_LEVEL could contain values such as:

GRADE 7
GRADE 8
GRADE 9
GRADE 10
GRADE 11
GRADE 12
LEVEL 1
LEVEL 2
LEVEL 3


You might consider adding a partitioning attribute to the GRADE_LEVEL table that would indicate which educational system is being applied.  This may be helpful for some of your application logic that might need to apply different processing depending on which system is in use for a given COURSE.