You have something close to the core of it there, because something like the subquery in the EXISTS expression is a good starting point. But it's going to print TRUE if any prerequisite course is passed, even if some prerequisites are failed or not yet taken.

The first thing to do is to throw away everything but that EXISTS expression; then change EXISTS to NOT EXISTS (as per your comment) and change

G.Grade >= 2.0 to IsNULL(G.Grade,0) < 2, because you will be using that clause to check for prerequisite courses which were failed or have not been taken, and change the subquery so that it begins SELECT 1 instead of SELECT ReqCourseID, G.Grade because you are just checking for existence, not returning values.

The second thing to do is to make It look at all the courses which might be the student's next, and list only those for which he is qualified: so in front of the NOT EXISTS clause you write SELECT @StudentId, C.CourseID from Course C WHERE and in the subquery change @NextCourseID to C.CourseID. Now you will have something that lists all the courses the student is qualified to enter; but it has a flaw: it lists courses the student has already passed as well as ones he hasn't yet passed. So you need to add AND followed by another NOT EXISTS expression to the WHERE clause. The new NOT EXIsts expression has to have a subquery which will select courses that the student has already passed. And that's it.

Once you are there, it is fairly easy to change the outermost select so that it uses StudentID from the student table and replace @studentid in the query by studentId, then add an ORDER BY StudentID, CourseID clause to guarantee that all data for a student comes out on adjacent lines rather than being scattered amongst data for other students; or to add some pretty formatting if you want it.