I want to add two things:
1- I want to assign a number of hours for each course and a max number of hours which allowed to any student. When any student try to enroll himself to a course the system must calculate the amount of hours which has taken by this student and refuse this enrollment if it reaches above the maximum hours.
2- I want to assign a certain time for each course to do some activities in (weekly activities) . When any student try to enroll himself to a course the system must refuse this enrollment if this course's activities conflict with another course's activities which has taken by this student (of course hasn't complete it yet).