Further on the StudentSubjects table...

You have a one-to-many relationship between StudentSubject and ExamResults. Is that a true representation of life, in other words can one student, for one subject, have more than one exam result (mid-term, finals, etc.)?

If yes (which I suspect), the ExamResults table needs one more bit of info about which exam this is. You could 1) add an ExamNo field in it; 2) add a table Exam with an ExamCode PK, and then an ExamCode FK field in ExamResults; 3) add an ExamDate field in ExamResults. Personally I would go for option 2 and list all possible exams in that school with fields like: ExamCode (PK), SubjectCode (FK), ExamDate, Building, Classroom, etc.

In any case, I agree with Aaron's suggestions above: remove StudentSubject table and replace with the modified ExamResults, make sure every table has a PK, and reinforce your constraints.